Okay, I hate to be a Negative Ned, but don't do that, please.
Why? For every web donation, the campaign has to pay a transaction fee to the company that processes the charge. This donation will eventually be reviewed and refunded IN FULL, which means the money they paid for the processing fee comes out of other donations.
And, not only do they pay the transaction fee when you initially donate the money, they also have to pay the fee AGAIN when they send it back to you.
Stop wasting the campaign's resources.