The best approach here is to get your SaaS application listed in the Azure AD App Gallery - see: https://video2.skills-academy.com/en-us/azure/active-directory/develop/v2-howto-app-gallery-listing
Our integrated gallery applications have schemas/mappings customized to fit the SaaS app's needs. The generic non-gallery app SCIM connector has a default set of attributes/mappings, and those will always remain the same and can't be automatically changed. It's possible to update the schema/mappings for an existing provisioning configuration via MS Graph and just hand the customer a schema that they need to upload, but I think that's likely more work than having them add the objectId -> externalId mapping in the UI. In any case, reference docs on our API for management of the schema is here: https://video2.skills-academy.com/en-us/graph/api/synchronization-synchronizationschema-update?view=graph-rest-beta&tabs=http