This is because the Page is not using the correct SSL setting.

To change this navigate to:
  1. onMessage
  2. Website
  3. Site Name
  4. Find the Page the Custom Form is placed on
  5. For Podium Site click the Security Tab, for onMessage Site click the Edit Settings (Gear Icon) 
  6. Select the Radio Button for "Force page to use SSL​"
  7. Save

Note: After making the change,wait a few minutes and then try to access the form again.