Contact Customer Care and ask to work with an Experience Cloud SSO specialist. They will work with you to set up your Experience Cloud Organization for SSO. Once that's setup, you can have Customer Care disable the legacy login for DTM. This will force all users to log into your Experience Cloud organization in order to access DTM.
While you're at it, you can also have Customer Care enable user and permissions management from within the Adobe Admin console. This will allow you to create DTM permissions groups/roles within the Admin Console. You might already be doing this for some of your other Adobe solutions.
If you're new to DTM, these options are enabled by default. If you're unsure, you can always try logging into DTM directly via dtm.adobe.com. If you receive an error message indicating you must log in via the Experience Cloud, you'll know it's enabled. You can also test your user and permissions management by visiting the "Users" tab in DTM. There you will see a message indicating permissions are managed by the Admin Console if the setting is enabled.