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You can do that by following the steps in this guide: How to pass a calculation to a payment field
Now your current form (https://secure.jotformpro.com/Hyogagroup/hys80dinner) already holds some products and to use this option, you would need to disable the products first and set it to donation.
The good thing is that you can set the similar options on your form just as you had as your products and then pass those values over as well.
Of course, if you have any questions along the way do let us know, but I would suggest to have us help you with your previous thread first, then it would be easy to just follow up on this thread as well to complete it all :)