If you have not read, see my list of items to prepare for appointment.
https://connect.mayoclinic.org/discussion/your-tips-on-how-to-get-off-to-the-best-start-with-a-new-specialist/?pg=1#comment-240765
This web page has a phone number for help with portal:
https://profile.mayoclinic.org/help?applicationId=c77283b6-1f64-4808-a8af-7bbc3c45595b
Even after you get registered for Portal, I think you can only send messages to dr you have already seen.

Prior to appointments at Mayo, I usually get a form to fill out as preparation for appt.

If you have medical records you want to send prior to appt., contact the person who scheduled your appt and ask about sending medical records or introduction letter. If unsure who to call, try calling central appt desk at 507-538-3270 and they can transfer to right appointment desk. If you have received a hard copy confirmation of appointment, there might be phone number on that.

Good luck
Laurie