Whatever your standard means of communication is. Don't make such a big deal out of it. It'll hardly depend on how you ask. She'll either agree or she won't. Just straight up ask her.

Full disclosure:
I recently asked a close friend that rejected me a few weeks before, if she'd just fuck me so I could get this virginity thing over with. Via chat. Amazingly, she said yes, so my advice is strongly coloured by this.