I’ve gotten a rash of blow job questions recently, mainly to do with how to get her to let you come in her mouth. The husbands wanting that, but very much not wanting the ruined orgasm from her pulling her head off and stopping all stimulation a quarter second before he cums.
Personally I would talk about it outside of the bedroom. Just say that it’s something you would like. Let her know that you don’t expect her to do something she hates, just ask for her to try. Explain that your orgasm is ruined if she pulls off as you cum, so it’s either switch to handjob at about T-minus two seconds, or don’t stop.
Absolutely do not trick her by screwing up the warning and trying to cum in her mouth as a surprise. That will set you back about two months at a minimum.
Act like you are really into it and loving it. If you’re basically holding your breath and silently hoping that tonight is the night, it’s not really that hot of an experience for her.
Try the moaning and groaning thing with some excited pawing at her as it gets better and better. Pull her hair a little in the 10-15 seconds to go mark. If it’s hot for you, it’s hot for her. Maybe she will, maybe she won’t. Just enjoy what you have in the moment.
If she does happen to let you cum in her mouth, don’t get all worked up about whether it was a spit or swallow experience either. Just say it was great and you loved it and love her.