For a standard mortgage (360-day interest calculation) there are two ways to do this: One way is to transfer an amount that includes both your regular payment plus the extra you want to put on principal. When the confirmation message appears, you will see exactly how that money will be applied to principal and all you need to do is confirm that the amounts look right. The second way is to transfer any amount you wish, then when the confirmation message appears, choose the option to put all of the money directly on to principal.