1. Go to Communities > Forms > Registration
2. Hover over your form name
3. Click Website Info
4. Click Registration Form Options
5. Click Next until you arrive at step 5 of 5
6. Click Advanced Confirmation Setup
7. Click on Use Advanced Confirmation Setup
8. Click Edit next to Registration Confirmation
9. Select Use Customized Text if it is not already marked
10. Click Edit Email
11. In the pop-up editor window, click Load Default Email
12. Place your cursor where you want to display the custom event field
13. Click the "eF" icon
14. Select the Custom Event Field you want to have fill in on the email
15. Click Submit
16. Click Save
 
Ensure that you go back into the Webinfo Checklist and click Update Your Event Now for these changes to take effect.