To apply a promo code, follow the following instructions in your Arch dashboard -
Click on the user icon in the top right of your Arch dashboard
Click on Account in the dropdown
Click on Promotions and enter your promo code
The promo code will now automatically be applied on your loan.
If you have more than 1 more promo code, go to this screen to apply the one you wish to use. When you go back to create a loan, you will see the promo code already applied.
