To use the value of a field as a discount voucher code, click on the field in the form editor, then click on the 'Processing' tab of the field properties on the right hand side, and select 'Discount Voucher Code' from the dropdown list for the setting 'Order Value'.
This will not work on quote request forms, because no payment is ever taken when a quote request form is submitted. It can be used on order forms and nCart checkout though. The only way of allowing for voucher-code based discounts on quotes is to setting 'Available for Quotes/Invoices' to 'yes' on the discount record (of course you can also incorporate discounts as line items or section discounts on the quote itself if you wish).