To add voucher-based affiliates in Xola, go to your Affiliate Commission report and click on the green "Create an Affiliate" button. Enter the affiliate's information in the pop-up box as prompted. Make sure to check the box labelled "This affiliate requires a voucher during booking." This affiliate will now be able to enter a voucher code and use a voucher as a form of payment during the booking.
Note that this can also be done on the fly: the same pop-up box will appear when you add an affiliate on the fly. See more information about adding affiliates on the fly here.