[Transfer ownership] Target user input only accepting username (without assistant)

[wiswedel]

The target user input field needs to be filled with a username (user ID) - which neither the initiator can know, nor any user ever gets displayed to themselves.

Some assistance with the target user choice (just like with the normal share menu) would help picking the right target.
Otherwise the target can never be found out about without asking an admin to look up the mapping.