Automation is a good idea! I just tested this and you can actually use an existing automation, if you’re willing to create a user custom field and require your users to complete it at signup.
Steps that worked for me:
- create a custom user field and make it required
- create a topic in your Staff category
- create a new automation
- choose “Create a post” script
- choose “After user update” trigger
- choose your custom field
- select “once per user”
- select “new users only”
- specify system as the creator
- specify the topic ID of your topic (the number that appears in the URL)
- write the content you like, including the variable e.g. “{{updated_user_username}} signed up!”
- sign up a new test user to test