Create your form and add a Number field, which will represent the donation box, and set it as required (it is a donation form after all, right?).
In Payments → Assign Values or Formulas to Fields set your Number field with the value 1. In Summary and Discounts multiply the Number field with 1. Now people will be able to donate from 1 dollar, euro or whatever the currency you have set up.
You cannot use the recurrent payment option with donation forms. The two options are incompatible one with another.