This API allows for the creation of users in an administrative and non-interactive way. This is generally used for bootstrapping a Synapse instance with administrator accounts.
To authenticate yourself to the server, you will need both the shared secret (registration_shared_secret in the homeserver configuration), and a one-time nonce. If the registration shared secret is not configured, this API is not enabled.
Here is a zip with the files to add SharedSecretRegistration as defined by https://matrix-org.github.io/synapse/latest/admin_api/register_api.html
gomatrixplus.zip