FMS can only do authentication with:
1) native FM accounts
2) Windows Active Directory accounts & groups
3) Apple Open Directory accounts & groups
4) Local accounts and groups in the OS of the FMS machine itself
5) Microsoft Azure AD accounts and groups
6) Google accounts
7) Amazon accounts
Anything else is not possible unless you first sync up with any of the #2 to 7 of that list.
Keep in mind that the LDAP settings on FMS *DO NOT* come into play for authentication. Those are just for finding an FMS in a busy/complex network.