You could use the CB Auto Actions incubator project with a Query action to insert into database a new forum post. This would allow access to substitutions such as [name], [username], [email], [cb_myfield], etc... I recommend using the after registration trigger in your scenario. The Kunena action just creates categories; it does not create forum posts.

Incubator projects come with no documentation. Nick however has been providing tutorials (not all for CB Auto Actions of course) below, which you're welcome to browser through.

www.joomlapolis.com/support/tutorials

Please provide a screenshot of your current configuration of the auto action and will review its configuration to see if there's any problems. Often culprits of an action not firing is Access or Condition misconfigured. Aside from that a query action just executes the database query you supplied.

Please remove the trailing comma on your Trigger. Once done ensure you're testing on frontend through normal registration workflows. The trigger specified is for frontend only. So if a user is created from backend it'll never execute. Please also ensure CB Auto Actions is published and public within CB > Plugin Management.