How to add an other field to payment section in my form?

I would like to add other to my products so people can make a doantion how do i do that? 

I think you could add a new product and set $1 as price, then add a quantity text box where your users will need to enter the amount they want to donate.

Here is also the link to my demo form where you can see how it works: https://www.jotformpro.com/form/63196152966970 

If you have any question, please let us know.