It is not documented. Best way would be to use a browser and use the Developer Console (F12) and see what calls it does.
An other option would be, and that might be a better solution for you is to use the Bitwarden CLI, which has a built-in API Server.
See: Password Manager CLI | Bitwarden Help Center
Hey, sorry for reviving this old post. I am trying to automate new user invitation with the Vaultwarden API. How do you find the token to authenticate?