This is currently possible using the automation plugin, though there’s a catch in that the automation would need to be triggered by the user filling out a custom user field on signup (it uses the “after user update” trigger).
So for example, on my test site I have an “accept the terms” checkbox on signup that’s required
And I have an automation configured to trigger when a new user completes this field
so then, when the new user completes this field on signup it posts to a topic
