Giving one of two options

I am really new to all of this, but I’m trying to edit a column with the following:

=OR(LEFT([Ticket Number],6) ="TASK00",LEN([Ticket Number])=0) 

where the user is required to enter their ticket number as “TASK00XXXXX” where X is the number of the ticket. I would like to adjust this to limit them to both TASK00 and RITM00 but I’m not having much luck. Any help is much appreciated!