If the custom form is a public form, you can add the form to a Test landing page:
  1. Go to onMessage
  2. Select Website > Main Site
  3. Under Pages & Menus, on the right click + Create Page
  4. Title the page, and for the location choose Landing Page
  5. Click Save & Close
  6. Click the wrench icon for the page and choose Edit Page Content
  7. Click +Add Row in any Page Content area
  8. Adjust the column size if necessary and click Save
  9. Scroll down on the left hand side of the page
  10. Drag Custom Forms and drop it in your newly created row
  11. Choose your form from the drop-down menu
  12. Click View Page

If the custom form is a secure form, you can add it to a Resource Board post:
  1. Go to Core 
  2. Select Communication > Resource Boards
  3. Choose a Resource Board
  4. Click +Post
  5. Add a Title
  6. Choose Detail Page
  7. Set the Publish Date to a future date
  8. Click Save
  9. Edit the post
  10. Click the Content button in the top left
  11. Drag the Forms List content type onto the page
  12. Click the Gear icon
  13. Mark the Custom Form
  14. Click Save
  15. Click View Page