You will need the Full Write Forms permission to perform this action.
Select the Donation Field
After adding a form, you will see, listed in the Add Fields section, several design choices. Among those is the Donation field.
Clicking on that "Donation Field" will display a pop up (below), where you can input the Label (field's name), a Description, if the field should be required and/or admin only, and you can select the field's designation or designations and if reoccurring donations should be included. After all of the information is put in, click on the "Done" button.
After completing the Donation pop up, you will see the field displayed on the form page.