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You could create multiple versions of the payment tool and use conditions to hide and show them based on the content of the text field. If you have trouble configuring this I can try to assist you. Please do remember though that there is no real support for promo codes so it will be hard to manage them :)
We do not have such feature. However, here's a workaround provided by my colleague on the other thread.
You can create an additional form with discount prices, and then redirect your customers to needed payment form using the following condition.
Or create a Show condition to show Free Text (HTML) field with link to discount prices form based on Promo Code Text Box field.
You can Clone mentioned above form to see how it is made.
Please feel free to contact us if you need any further assistance.
EltonCris's solution is less messy. You should probably use it.