What Xojo Release version ?
What macOS or OS X version ?
I checked with Xojo 2016r4.1
El Capitan 15G1217
Also:
Youd betterquit all applications, power off, then power on, run Xojo, open your project click in App.Icon and do your drag and drop.
I have mixed results with Xojo 2015r1, but I have a tons of running applications and I am working without power off, etc. since
during 5 hours npw.
The clicked Tab may vary your experience too.
Nice. And do you compile for Windows too ? If so, how do you do that for Windows ?
If it makes y’all feel any better this is just as difficult in Xcode.
Well, if the icns already exists, it is much more rational to place it yourself in the bundle with a copyFile step, than to drag it over the icon editor to add more noise to the signal.
BTW I am not sure this editor would be astute enough to assign the proper size to each box.
I usually start with a 1024x1204 picture, and reduce with gaussian sharp in Photoshop to get the best result at smaller point sizes, especially 32x32 and 16x16 where the Xojo editor would have a very poor rendition. For some apps when reducing to 16x16 would not do, I even changed the design of the icon.
If you read the specs for a ICNS file you’ll find that some sizes are PNGs with full transparency - so there’s no mask required
Nor is one imported as this would actually make your image look worse to split it into IMAGE + MASK and then recompose it into a PNG with transparency
Some sizes are IMAGE + MASK in the ICNS file itself and are imported as such
And it does split them all into the right sizes etc since each item in the ICNS file specifies that TAG X means 1024 x 1024 PNG etc etc etc
And for Windows programms ? What is the equivalent of the .icns CopyFiles step on Mac ? 
Think different ?
View different ?
Take the question from a different point of view
What was before the icns ?
If you have the icon as different sized image files, it may be a better idea to forget the icns drag to the App Icon at all and use the original sized images files.
icns is a macOS file. Not related to WIndows.
That said, the essence of the question is important: what is the procedure to store the application icon using CopyFiles step on WIndows ?
Yes, you’re right : so what is the procedure to store the application icon using CopyFiles step on WIndows ?