Drag any file onto this button - you'll get the red crossed circle icon that is shown when a drop is not possible. This is not what should happen in my opinion.
Edit the button and add any number of characters into the Parameters field. I used a single space character since that doesn't interfere with the command itself in any meaningful way.
Drag a file onto the button again - now the drop is possible and Notepad will be launched with the dragged/dropped file.
To me it looks like when Parameters were added to em_ commands (in TC 11.0?) the Drag'n'Drop feature was left out by mistake; I don't think that it's intentional.
Regards
Dalai
#101164 Personal licence
Ryzen 5 2600, 16 GiB RAM, ASUS Prime X370-A, Win7 x64
This happens because buttons with parameters are treated differently from buttons without them: Those without can hold multiple commands, e.g. cm_focusleft,em_notepad which isn't possible when there are parameters. In your specific case, TC would have to analyze the command more before deciding whether drag&drop is allowed or not. I will check whether this is feasible.