Registration to MSAD with client certificate user ID

I was able to get x509 client certificate authentication working with MSAD, but I am having an issue with new user registrations writing back to MSAD with the needed ID pulled from the client certificate.

Currently mapping subject alternative name (UPN) from the certificate to the userPrincipalName in MSAD to do the authentication. However, when writing back to MSAD for new registrations, how do I write that UPN information pulled from the certificate back to the userPrincipalName attribute in MSAD? Is there a special user model attribute that ends up storing this information that I can map back to MSAD?