I reach out to the girl and ask her if it’s OK if I use her as a reference with her booker. Then I tell the booker she’s expecting him to reach out to her.  

This works every time so far.

From very trusted mongers the procedure (obviously can slightly vary) is the following:
You talk to her and give her the name/number etc and ask her to validate you with PO. Then you text the PO and say that she was supposed to validate your number.  

Note that some orgs, for example bay area LSC, are very strict and will not allow it without regular screening anyway. So this will not work wit them even if girl is your atf you've seen 50 times and she loves you.  

 
Me I never bother to get girls numbers. Some pos are amazed that I go dozens of sessions and don't have the girls contact lol.

And I have had varied levels of success with this. The problem with booking with the girl directly is you’re trying to make your appointment time fit in with the other bookings coming in from her booker, and while you may not give a shit if someone else’s appointment is being moved around, it still can complicate things with the booker, and by extension the org.  

That’s not to say this is always an issue. Sometimes it isn’t an issue at all. In fact, it almost never is an issue. Though, I don’t recommend it. For the aforementioned reason.  

Luckily from what I’ve heard girls aren’t offering this option as much anymore in the current era of the hobby. Wouldn't make much sense and it isn’t necessary