From the screen’s Actions tab, choose Transfer, pick the destination workspace, and confirm.
What you need
Owner or Admin in both workspaces. The destination list only shows workspaces where you have one of those roles, so a workspace you cannot see in the list is one you do not have rights in.
You also need a spare license in the destination.
What comes with it
Settings come across. Playlist assignments do not.
That is the thing worth planning for. A transferred screen arrives configured but with nothing to play, so line up its playlists in the destination workspace before you move it rather than after.
The screen also arrives disabled. Open it in the new workspace, enable it, and it starts playing.
Why you would
Workspaces are isolated: content and screens in one are invisible from another. Moving a screen is how you hand a display to a different team, or split a growing fleet into regions.
If you find yourself transferring screens back and forth often, that is usually a sign the workspaces should be linked or that a shared playlist would solve the actual problem.