Unless I’m misunderstanding what you are trying to do, you will probably need a custom action. This will create a comment transaction in response to the “On Create” condition. The template can probably be “Blank” as the custom action will contain the Perl required to create the new comment, including the text of the comment.
The only major “gotcha” I can see is that you want to use “On Create” so the ticket may not actually exist fully in the database yet (it is sort of half created inside an atomic SQL Transaction that isn’t committed until after the scrips have run). I’m not sure if that will mean that doing something like
$self->TicketObj->Comment(Content => 'Hello'); in a custom action will fail or not.