CAN I OFFER A DONATION FORM WITH A OPTION OF OTHER AND THEY FILL IN THE AMOUNT?

https://secure2.convio.net/bgca/site/Donation2?7942.donation=form1&df_id=7942&clickDonate=false

SEE THE LINK ABOVE

There are two ways on this:

1) You can set up a minimum fee, put a fixed price, or leave it your visitor.

Check out this form template and even clone it to your own account to get a quick start on setting up your donation form.

https://www.jotform.com/help/165-Setting-Up-Your-First-Order-Form

2) If you want to re-create something similar to what you have shared: 

Here is how:

a) Add a Radio Button fields from Form Tools

b) Once you add the options, click on Allow Other

c) Add the Form Calculation widget, click on Add Field, and add the Radio Button fields, as shown in the animated image bellow:

d) Hide the calculation widget field:

e) Follow this guide to pass the total calculation to you payment field: https://www.jotform.com/help/275-How-to-pass-a-calculation-to-a-payment-field

Hope this helps, let us know if you have more questions, we will be glad to assist you.