Basically you take the video or photos yourself and let the porn star know what you want to catch (on video/photo). Some will require a higher donation for this type of souvenir, others won't. You generally have to ask before the appointment but some porn stars are cool as long as you ask first. As for indy providers, if they don't mention this in their ad or website don't even attempt to ask about it. Remember discretion is utmost important to indy providers.
As others mentioned you would either do a POV or have a tripod setup with the camcorder recording if you want your hands free.