This may be done by using a merge field within the Advanced Confirmation Email.
  1. Navigate to Communities > Special Events > Friends Asking Friends > Kintera Thon.
  2. Hover over your event and click Webinfo Checklist.
  3. Click Event Text.
  4. Scroll down to the section that says: Registration Confirmation Text and Details
  5. Click on Advanced Confirmation Setup
  6. Select the radio button next to Use Advanced Confirmation Setup
  7. Click on Edit next to the Registration Confirmation
  8. Select the radio button next to Use Customized Text
  9. Click on Edit Email - this brings up the Confirmation Editor
    • If you do not already have an email in place, click on Load Default Email
  10. Click on the area that you would like to add the Additional Donation Amount field
  11. Click on the event field cylinder (eF)
  12. Mark the box next to Choose Additional Donation Amount and click Ok

You will now see a merge field that says [[additional_donation_amount]] - this is where the amount will display.
You may wish to add text before the merge field to say "Additional Donation Amount:"