Synapse Home Server

I am trying out the Synapse server for Element.

How do I register a new server admin? I can reach homeserver.db by SSH, but how do I manage it?

I tried to find the API for this, but they only suggest register_new_matrix_user , but then I have to make server commands and how do I even reach the command prompt for the Synapse server then?

Seems like I almost manage with the API, but I can’t find how to generate the MAC.
It gives me the following error code:


"errcode": "M_UNKNOWN",

"error": "HMAC incorrect"


How can I generate this one from Umbrel?