How do I access the File System API of emscripten when compiled with MODULARIZE option?

If I compile my C++ code with emcc without -s MODULARIZE=1 -s 'EXPORT_NAME="createMyModule"', I can load my WebAssembly and access the File System API like this:

HTML:

<script type="text/javascript">
  var Module = {
    onRuntimeInitialized: function() {
      console.log('Module loaded!');
      if (FS.mkdir) {
        console.log('File System API available!');
      }
    }
  };
</script>
<script type="text/javascript" src="myWebAssemblyModule.js"></script>
<script type="module" src="my.js"></script>

Log:

Module loaded!
File System API available!

But if I add -s MODULARIZE=1 -s 'EXPORT_NAME="createMyModule"' as compiler options, and try to access the File System API based on a Promise, it doesn't work:

HTML:

<script type="text/javascript" src="myWebAssemblyModule.js"></script>
<script type="module" src="my.js"></script>

my.js:

createMyModule().then(MyModule => {
  console.log('Module loaded!');
  if (MyModule.FS.mkdir) {
    console.log('File System API available!');
  }
});

Log:

Module loaded!

So how can I access the File System API with the "MODULARIZED/Promise based" approach?

MyModule.FS is accessible btw., but not MyModule.FS.mkdir.