An administrator wants to create a list that contains all prospects who complete the Contact...

Use an automation rule to automatically add or remove prospects to the list if they submit the form, but aren't opted out.

Use a dynamic list to automatically add or remove prospects based on the form completion and their opt out status.

Use a completion action on the form to automatically add anyone who completes it to the list.

Use table actions to add prospects to the list if they've filled out the form, but haven't opted out from communications.