You should message the mentor with any additional specifics about what you want to discuss. If you have a website or any docs, send it on over so they have an idea of what you working on.
Give the mentor as many details as possible. They LOVE details!
Please keep in mind that the mentor is not obliged to do any work outside of the scheduled session time. Generally mentors are happy to get as much info as possible before a session so that they can plan accordingly.
It is a good idea to add your new contact on the chosen communication tool in advance to make sure that you are ready to start at the scheduled time.