If an agent creates a new ticket, in most cases he is also the owner of this ticket. The owner should be preassigned (vorbelegt) with the current agent.
In the macros, the action to assign the current user as owner, isn’t possible in core workflows.
Did you also check the auto-assignment feature?
Also trigger could work for this use case.
I think with core workflow this is currently not possible, auto select is here for optional fields but has only one existing option in the select list.