You can add a Select One field by adding another field and setting it as the default. To do this, you will go to:
  1. onMessage > content > custom forms
  2. Edit the appropriate custom form
  3. Edit Form
  4. Edit the appropriate section
  5. Enter Select One in the Add Value text box
  6. Click Add
  7. Mark Default next to the newly added Select One field
  8. Save