How Do you add an other option in the payment gateway

New question for support On a donation form that provides options to the
donor for amount levels, How do you add an other option that allows for a
box where the donaor can add their own amount?

Though it is not possible to have both products and a custom amount option in the payment field, it is possible to use a workaround to include both in the form and pass that value to the payment field.

#1. Instead of displaying the membership options in the PayPal question, you can add the products in a check box question.  You can assign those products calculation values:

http://www.jotform.com/help/301-How-to-Assign-Calculation-Value

#2. Add a text box to collect the donation amount. 

#3. Add a "Form Calculation" widget in your form to calculate the total amount of membership and donation.

http://www.jotform.com/help/259-How-to-perform-calculation-in-the-form

#4. Add PayPal payment question in your form and set it to read the amount from the calculation widget. The following guide should help you how to pass calculation to PayPal payment question:  http://www.jotform.com/help/275-How-to-pass-a-calculation-to-a-payment-field 

Please check the screenshot below for all the steps: