I know when signing up to 2d there is an option to add your pay monthly number, but what about after you've signed up?

We just signed up to 2d a few weeks ago (scheduled to go live later this week), but at the time weren't on pay monthly, but have switched a pre-pay account over since, but can't find any option in the broadband accounts page on adding a 2d pay monthly mobile number, to get the $10/m discount.