How do you structure your lease-up commission with leasing agents for recently completed multifamily buildings?

when you finish a project and begin the lease up process, what arrangement do you have with the brokerage firm and their agents? if its OP, do you come to a predetermined fee? is each apartment leased a fixed fee to the firm? are there bonus hurdles?  what about paying outside brokers?

any insight into this would be appreciated. Thanks!