Yes, email is probably the best way to go, as other have already suggested.
That being said, if you are communicating with her for the first time, I would avoid any detailed and pornographic description of your fantasy in your first email. Keep it professional and G-rated, explaining that you have a particular fantasy/role-play in mind, and asking her if it is OK with her to share more details. Just give enough details to give her an idea of the kind of fantasy/role play you are thinking about (for instance, Dom/sub play, humiliation play, secretary/boss play, foot fetish, etc.). She will let you know if this is within her range of services, and if she is interested in hearing more about it.
For many SPs, receiving a first email that reads like an erotica story or pornographic script will send the wrong vibe.
If the lady works for an agency, in my experience, the best way to go about it is to meet her first for a "regular" session, and bring up your fantasy before or after the festivities. Depending on your fantasy, she may be able and willing to indulge you in that session. If it asks for more preparation, then she can probably give you a work email address to communicate with her for a later date.