Adding markers in play mode is straightforward. Click on the timeline and then alt-shift-M. But in write mode it is not straightforward. Markers are placed where the playhead is. But to get the playhead to a specific beat you need to select a note, then press play and then quickly press pause (before play actually starts) to advance the playhead before adding the marker. Otherwise it puts the marker at the previous playhead point.
So, unless I am missing something, it would be more useful to add the marker in write mode at the current selection rather than the playhead point. Or provide a way to move the playhead easily.
Also, markers only show up in play mode when a video is attached. But they show up in write mode without a video attached. This is inconsistent. I would possibly find it useful to have markers in write mode without a video, but I think this is not the current intent.