It is possible to add an empty user which can then later be updated via the returned user Id.
You can use the externalId to connect a user to the unique Id of your system. It is not assumed to be unique.
SsoUserId is the single sign-on Id of the user. In this way the Kazi backend is linked to the single sign-on system. In Azure you can find the SSO Id in the id_token under the name 'oid' (objectId). Read more about SSO here.