When trying to add new members, Neo member management doesn't have the custom identity provider configured in the global account, as the user base.
For subaccounts in the Neo environment, the identity provider will be offered in the value help only if a user in that identity provider has created at least one Neo subaccount in the corresponding global account and Neo region. For more information about this, see here.
A user from the configured custom identity provider needs to create at least one new Neo subaccount in the Neo region where the user base selection is missing.