I’ve tried to follow this documentation to drag a Windows .ico file and populate the Images and Masks for the App icon:
"You can also drag an ICNS or ICO file onto the window background (instead of an image area) to load and populate all the image sizes at one time. If you add an icon that has both the image and the mask (such as with most PNG files) then both the image and mask are added. "
This just will NOT work. No attempt to drag will result in a drop.
Can you share the Windows and Xojo versions ?
Yeah, dropping the image or .ico there won’t work.
Double click on the app icon to open the icon editor. Right click on any of the image squares and select import from the drop menu.
Not only won’t works, but it crash the IDE.
They started to implement drag and drop, but sometime in the job, they forgot to complete the code, so the Icon Editor does not Open itself and do not display the dropped image 
so it crash.