If you want users to be redirected to a specific page after completing a Form, you can use a return URL.
Go to the Online Registrations section from the Dashboard.
Click the link icon on the Form you would like to set a return URL.
Click to enlarge
A small text box will appear beneath the words "Create a link with a custom return URL..." Enter the URL of the web page you like the user to be redirected to after completion of the Form, then click Create Link.
This will modify the link below to accommodate for the return URL. Use this link to link to the Form from the Website Builder.
Note that the Online Form widget does not accommodate for return URLs - you must use the link instead.