Your CMS Cloud forms have an option to include a Donor Covers Fee check box which gives the donor the option of covering your transaction fee. You can add this option to your, and what the fee percent is.
You can also add a custom message for the check box.
To add this option, simply drag the "Cover Processing Fee" element onto the form. A good place for this is right below the Amount, or above the Submit button.
Once this element is on your form, you can edit the processing fee and set the message.