How to include a emscripten sub project by using add_subdirectory

[Roy-Kid]

Hi, I have a c++ project and I am going to bind it with both javascript and python.
I write binding code with embind and as a sub directory in the project.
I use add_subdirectory to include it into my building system. Can I do this?
I think I should use emcmake, but it seems can only use outside.
I try to set PATH manually but not work...

How should I do?