Well if you are really serious about asking for extras and not just a sleazy punter, or a cheap time waster, then you have to put into song and dance.
All girls love musicals. Gays do too, but that's for a different forum. Lets stick to girls.
So you have to compose a song and sing it just like in a musical.
Learn from the masters, Gilbert and Sullivan, Rogers and Hammerstein, Andrew Lloyd Weber, Leonard Bernstein, Steven Sondheim etc.
And then you have to sing it.
And if you are really serious then choreograph a dance routine for it too. Learn from the Masters like Gene Kelly, Fred Astaire, Sammy Davis Jr, and Frank Sinatra.
She will be totally charmed and she will suck your dick. And if its a sexy song and you sing and dance it well she will let you put your dick into her tight little pussy, and then you can sing a beautiful duet together, just like "Tonight" in West Side Story, but not "Somewhere" that's a bit too sad,