Coupon code form field redirect to alternate payment page

[David4321]

Can anyone provide a snip of code that I could insert into the "coupon code" field on a formmail form which has the following effect:

 

on submit form, user goes to the normal payment page (this is already implemented)

if coupon code field is filled out with correct code, user is taken to alternate payment page on form submit.

 

Either this or another approach which has the same result is OK. I am a novice coder working in a wysiwyg - I don't REALLY know php at all - I just need a snip I can insert and a clear instruction.

 

Thanks so much for any help.