I'm assuming you're looking more for of an escort session than a massage provider. RentMen conveniently has a "Into" option where providers can indicate that role play is an option, and it's easy to search on that so you can identify some guys to contact first.
As far as requesting it, I agree with @Jamie21 that it is probably best to keep the scene relatively simple. You can message them and say simply that "I am interested in a roleplaying scene that starts with a massage and transitions to an X type scene, can you do that?" They may agree readily, or suggest a discussion in advance, depending on what you're asking for.
Also, if your roleplay scene involves gear, props, or anything that may require advance preparation, you definitely want let them know, so they can tell you if it's possible or not.