Sorting Registrants into Breakout Rooms before the Meeting Starts

I set up a Meeting with Registration.  There are 85 registrants.  I had created the 4 breakout rooms already.  but when I look at the Breakout rooms, it shows Zero participants in each and I am unable to add anyone from the Registration list.  In Registration, I had participants select which topic they wanted to be part of during the meeting.  I need to pre-assign all the registrants to the correct room before the meeting starts.  I am not seeing a way to do this after I already set up registration.  Any help would be great.  Thank you!