@mtanner242 

There are 2 ways that you can do this..

1st way: You probably made people who subscribe through this form subscribe to a specific list so you can use that list to start a flow. When someone subscribes through the form, they will subscribe to that list, than they will start a flow and in that flow you will send email with the Free Shipping code.

2nd way: This is better way in case you made that previous form subscribe to a main list or some other list for which this is not the only way to subscribe. For this way you will use a segment with definition either properties about someone - consent_form_id - add that form consent id (6 character code in the URL) or use the definition properties about someone - source - the source for this form (you can find the source for this form inside the form when you click on the button)