How do i make customizable buttons and fields with paypal?

i want to have a drop down box and a field to enter amount separate on a site my company is working on, but when the person donating the money chooses the options in the drop down box, and enters the amount into the field, and then finally clicks donate. I want it to automatically populate the paypal page with the information they had entered into the fields on the page before. would i have to just manipulate the code that was spit out from paypal, or is this even possible?