Contact...

You can just put Appointment in the subject line or inquiring on availability and specify the date.
I do not have any preferences most gents that contact me just put Appointment in the subject line.
Good luck to you.Also a short introduction in the email is a nice touch.

Then , in the body of the email -  few lines about myself ( so that she can do some preliminary screening),  where I saw the provider ( your ad on the ______ board, your post about _____, etc), what attracts me to her ( the way you wrote about yourself on your webpage, your photos , ______'s review of his time with you etc), and when I am intersted in an appointment.
With all that, I try to keep it to the point and brief.
Good luck

'A date' in the subject line.  Keep it simple, as she knows why you are contacting her.  In the body of the email then, I give her some info on me, maybe how/where I found her, when I would like to see her and for how long, and my reference information.  If it's a first contact, I will also ask if she needs any additional information.  Remember, do not mention anything sexual, if you want a reply.  If she wants me to follow up with a phone call, I will give her my number, telling her it's so she will know it's me when I call.

Swim

If you follow...

what wow and swimtrekr  said these two gentleman have provided perfect examples of the RIGHT way to send an appointment request.I wish every guy would initiate an appointment request like this or in a similar fashion it makes things so much easier.

Depend On If Your Serious, Or Just Asking Dumb ?'s...

I hate it when someone does not, do their own homework by referring to my website, or reading my reviews before contacting me it just shows your wasting my time.  If your looking to book every provider does not have the same method, so you should always do your homework 1st.

The next question will cost you.  LOL smile 4 me!