Capped Achievement in Halo 5: Forge
Attach a screenshot to your Forge Map.
How to unlock CappedNote: Due to a change in the way screenshots are handled with the old Xbox app and new, this is currently only obtainable if you have Halo 5: Guardians.
Launch Halo 5: Guardians on Xbox One and take a screenshot of anything, then upload it.
Then wait 10 minutes or so, go back to Halo 5: Forge on your PC and go to Forge > My Files > your map > Link Screenshot. Select the screenshot then attach and save.