The system won't be able remember why the user was created unless the content of the post includes data saying it was a signup. That's important for any type of reporting like telemetry and billing.

So then one gets to bike-shed if "signup" it is in the request path, query parameters, or the body. Or that since the user resource doesn't exist yet perhaps one can't call a method on it, so it really should be /users:signup (on the users collection, like /users:add).

Provided one isn't opposed to adopting what was bike-shedded elsewhere, there is a fairly well specified way of doing something RESTful, here is a link to its custom methods page: https://google.aip.dev/136. Its approach would be to add information about signup in a request to the post to /users: https://google.aip.dev/133. More or less it describes a way to be RESTful with HTTP/1.1+JSON or gRPC.