After users submit a form, the default behaviour is for them to be shown a confirmation message. However, after a successful submit you can instead forward them to a landing page, where you can provide them with whatever information they might need.
This immediate forwarding action can be defined in the settings areas of the respective form.
This section shows you how to set up forwarding after form submission.
You have now set up forwarding to an external website. When the form is published, users will be forwarded to the landing page that you entered after they have submitted the form.