How do I add a donation payment where the client/vendor can put in the amount?

Once you have added the payment integration to your form you will get a wizard where you will need to set up the payment type, here you need to select "Collect Donations".

This will be found under the "Payment Type Options": 

This way a text box will be shown to your submitter and they will be able to enter the amount they want to donate. 

If you have any question, please let us know.