You dont have to have a button, was just an idea if you wanted to be able to swap/change the ticket user for any reason - lunch break maybe or something like that.
If you went with ticket tag you would need an action to update ticket tag when ticket is created.;

A rule containing that action to set the value on ticket created using {:CURRENTUSER}

Then change your state format from T.User to TT.TicketUser (the name of the tag)