Sometime you may attempt to setup a form and receive the following message:
If this occurs, then it means you (or someone else on your Autopilot account) has already configured this form. To access your existing forms, click on a Form Submitted trigger, then click "Manage Forms":
Then on the Form Manager you will be able to see forms already tracked on that page:
Once you figure out the form you'd like to select, go back to the Form Submitted trigger and select it.