How to check a custom field value

Hi I would like to create a custom field for promotion code and handle the value with a conditional statement when the user push the submit button

in other words I’d like to create a custom field in which the user input a promo code. I’d like when he submit the form to check it against a list of valid promo codes and, if valid, processing the form

is it possible to do it?

the pro version can handle such a request?

There is thousands ways of doing it js/php and so on just use what fits you best.
I do not know which one is right for the platform though.
A database trigger comparing the value/values to a lookup table on save?
If you like to boost user level on the user in the same action it is possible if you kept wp user handling and just uses S2 as addon.
/Ice