How to set up donation payment field in payment wizard using PayPal?

In the PayPal integration, you can set the Payment Type to "Collect Donations".

Here's how it'll look like:

Please refer to this thread if you want to add an additional item in your existing products and then allow the user to enter their desired amount. Here's the link: https://www.jotform.com/answers/1018076

Hope that helps. Thank you.