Every time a ROI Form is created an Webform Trigger is automatically generated in Data > External Segments. The Webform Trigger will have the same name as the form. Webform Triggers can also be created without making a form.

You can see the Webform Trigger ID in the Form code:

When creating a new tactic within a campaign to send an ROI email, type the name of the Webform Trigger as the segment.

Now, every time someone fills out a form, the ROI email you select will be sent to the new form fills.