FormA has a multi-lookup to FormB, with ability to "add new"

FormB has an "existing relationship" lookup to FormA.

My expectation is that, from Form A, I can add new values in FormB - the problem I'm having is that:

I think FormB should automatically KNOW the value of the linking field, without my having to enter it, and it doesn't.

Should FormB be able to automatically know the value of the linking field, because of the bi-directionality?  Or is it really necessary to add the value manually????