"Let participants choose room" with "Breakout Room pre-assign" -ed

I've created a meeting with "Breakout Room pre-assign" enabled, with participants field is empty. My meetings is somewhat open, and it takes alot of effort to acquire email of said participants. I did this to avoid the hassle in creating in-meeting Breakout Room, because my meeting breakout room is quite alot.

When my meeting started, i (host/co-host) need to manually assign every participants to certain Breakout Room, with no "Let participants choose room" setting to be found.

Am i missing something or i failed to understand what "Breakout Room pre-assign" mean? Please help.

English is not my first language, pardon me if what i said is hard to understand.

Thank you.