As a Registration Manager:
  1. Navigate to onMessage.
  2. Select Approval > Event Registration.
  3. Click Edit to the right of a Registration. 
  4. Click the Extra Fields link on the left. 
  5. To the right of the Extra Field you want to mark Required, click Edit.
  6. Click the Required checkbox.
  7. Click Save or Save & Exit. 

User-added image