There are 2 ways we can achieve this :

1. Simply create a multi use promotion with appropriate coupon. Now in the postCreateUser method of ProfileFormHandler, just add this coupon to the user profile.
2. This can also be achieved through ATG Scenarios. Build an ATG Scenario which triggers an event on user registration. Use this event of the Scenario to create the promotion for dollar Off.

Hope this help someone else as well.

Thanks,

Mayank Batra