When setting up the self-service portal, it is recommended to use an identify provider like Azure AD. Self Service portals provide local login capabilities but this leads to password management requirements. Therefore, it is strongly recommended that local logins and user management functionality be disabled on the self-service portal so customers are able to use their Office 365 credentials to log into the portal. To do this follow the recommended site settings for the self-service portal as detailed here.
From here you still have the issue where customer’s created a local account to the portal - these steps may help with getting them to start using their Office 365/Azure AD credentials.
- Revoke the customer's portal invitation
- Send the customer a new invitation to the portal. See Inviting Customers to the Portal for details.
- Have them open the link in a private browser, and register for the portal again.
- Then they login their Office 365 credentials