matrixtoken

The Client-Server Matrix protocol allows to limit registrations based on
tokens, Synapse (and other Matrix servers) support this feature and
have an admin API to manage these tokens.
matrixtoken is a tool that allows users of the system to generate Matrix
registration tokens from the homeserver admin API, without being admins
themselves.
It is supposed to be used with the SUID mode to allow it to read its config
file containing the admin's access token. See the example configuration file
for all the available options.
After it has been installed and configured, users can simply run:
matrixtoken
Which will return a token with a limited amount of uses and an expiration
date (by default 1 use and 30 days of validity).