page-settings.php

<script>
(function($) {
    $(function() {
        var settings = <?php echo json_encode( $this->settings ); ?>;
        $('.pm-enabled').prop('checked', settings.enabled);
        $('.pm-api-key').val(settings.api_key);
        $('.pm-sender-address').val(settings.sender_address);
        $('.pm-force-html').prop('checked', settings.force_html);
        $('.pm-track-opens').prop('checked', settings.track_opens);

        $(document).on('click', '.save-settings', function() {
            var data = {
                'enabled': $('.pm-enabled').is(':checked') ? 1 : 0,
                'api_key': $('.pm-api-key').val(),
                'sender_address': $('.pm-sender-address').val(),
                'force_html': $('.pm-force-html').is(':checked') ? 1 : 0,
                'track_opens': $('.pm-track-opens').is(':checked') ? 1 : 0
            };

            $.post(ajaxurl, {
                'action': 'postmark_save',
                'data': JSON.stringify(data)
            }, function(response) {
                $('.pm-notice').html('<p>' + response + '</p>');
                $('.pm-notice').removeClass('hidden');
            });
        });

        $(document).on('click', '.send-test', function() {
            $.post(ajaxurl, {
                'action': 'postmark_test',
                'email': $('.pm-test-email').val(),
                'with_tracking_and_html': $('.pm-test-with-opens').is(':checked') ? 1 : 0,
                'override_from_address' : $('.pm-test-email-sender').val()
            }, function(response) {
                $('.pm-notice').html('<p>' + response + '</p>');
                $('.pm-notice').removeClass('hidden');
            });
        });

        //if we enable the track opens, we should also enable the force html.
        $('.pm-track-opens').click(function(){
            if ($(this).is(':checked')) {
                $('.pm-force-html').attr('checked', 'checked');
            }
        });

        $('.pm-force-html').click(function(){
            if (!$(this).is(':checked')) {
                $('.pm-track-opens').removeAttr('checked');
            }
        });
    });
})(jQuery);
</script>

<style>
.footnote {
    font-size: 13px;
}
input[type=text] {
    width: 300px;
}
</style>

<div class="wrap">
    <div style="background: #FFDE00; padding: 10px; border-radius: 5px">
        <h1>
            <a href="https://postmarkapp.com/" target="_blank"><img src="<?php echo POSTMARK_URL; ?>/images/logo.png" width="130" height="21" alt="" /></a>
        </h1>
    </div>
    <br/>
    <div class="updated notice pm-notice hidden"></div>
    <table class="form-table" style="max-width:740px;">
        <tr>
            <th><label>Enabled?</label></th>
            <td>
                <input type="checkbox" class="pm-enabled" value="1" />
                <span class="footnote">Send emails using Postmark's REST API</span>
            </td>
        </tr>
        <tr>
            <th><label>API Key</label></th>
            <td>
                <input type="text" class="pm-api-key" value="" />
                <div class="footnote">Your API key is available in the <strong>Credentials</strong> screen of your <a href="https://account.postmarkapp.com/servers" target="_blank">Postmark Server</a>.</div>
            </td>
        </tr>
        <tr>
            <th><label>Sender Email Address</label></th>
            <td>
                <input type="text" class="pm-sender-address" value="" />
                <div class="footnote">This email must be a verified <a href="https://account.postmarkapp.com/signatures" target="_blank">Sender Signature</a>. It will appear as the "from" address on all outbound emails.<br/><br/>
                You may override the "From" address set here on individual emails, by including a 'From' header with the address you wish to send from. <a href="#example">See the example below.</a>
                </div>
            </td>
        </tr>
        <tr>
            <th><label>Force HTML</label></th>
            <td>
                <input type="checkbox" class="pm-force-html" value="1" />
                <span class="footnote">Force emails to be sent as HTML.<br/><br/>DEPRECATED: Instead of enabling this feature, add a header to your HTML message with name 'Content-Type' and value 'text/html'. <a href="#example">See the example below.</a>
                </span>
            </td>
        </tr>
        <tr>
            <th><label>Track Opens</label></th>
            <td>
                <input type="checkbox" class="pm-track-opens" value="1" />
                <span class="footnote">Track email opens (which also requires emails to be "forced" to HTML).<br/><br/>DEPRECATED: Instead of enabling this feature, add a header to your HTML message called 'X-PM-Track-Opens' and a value of 'true'. <a href="#example">See the example below.</a>
            </td>
        </tr>
    </table>

    <div class="submit">
        <input type="submit" class="button-primary save-settings" value="Save Changes" />
    </div>

    <h3>Send Test Email</h3>
    <table class="form-table">
        <tr>
            <th><label>Recipient</label></th>
            <td><input type="text" class="pm-test-email" value="" placeholder="recipient@example.com" /></td>
        </tr>
        <tr>
            <th><label>Override Sender Email Address</label></th>
            <td><input type="text" class="pm-test-email-sender" value="" placeholder="sender@example.com" /></td>
        </tr>
        <tr>
            <td colspan="2"><input type="checkbox" name="with_tracking_and_html" class="pm-test-with-opens" value="" />Send test as HTML, with Open Tracking Enabled.</td>
        </tr>
    </table>

    <div class="submit">
        <input type="submit" class="button-primary send-test" value="Send Test Email" />
    </div>
    <div style="max-width:740px;" id="example">
        <h2>Overriding Sending Behavior for Individual Messages:</h2>
         "Forcing HTML" for Wordpress-generated emails (such as password reset emails) will cause them to be sent as HTML, this is often incorrect. Instead, individual messages should include the header above to have them treated as HTML instead of plain text

        <h4>Example Overrides:</h4>
        <pre>
        //Create a headers array:
        $headers = array();

        // Set this header if you want to override the default 'From' address:
        $headers['From'] = 'john.smith@example.com';

        // Set this header if you want the message to be sent as HTML.
        $headers['Content-Type'] = 'text/html';
        
        // Set this header if you want to enable open tracking for this message.
        // Setting this header also forces the message to be treated as HTML.
        $headers['X-PM-Track-Opens'] = true;

        // Send the email, including the $headers array we just created:
        $response = wp_mail( $to, $subject, $message, $headers );

        </pre>
        For more information on setting headers using the wp_mail function, <a href="https://developer.wordpress.org/reference/functions/wp_mail/">see the Wordpress Codex page.</a>
    </div>
</div>