The best way to configure a thank you email to be sent after the form on the "Request a Demo" landing page is submitted is to configure an autoresponder email to send as a completion action when the "Request a Demo" form has been submitted. An autoresponder email is an email that is automatically sent to a prospect after they complete a desired activity, such as submitting a form or clicking a custom link. You can use autoresponder emails to thank your prospects, confirm their actions, or provide them with additional information or content. A completion action is an action that Marketing Cloud Account Engagement executes after a prospect successfully completes a desired activity, such as submitting a form or clicking a custom link. You can use completion actions to perform actions on the prospects who complete the activity, such as adding them to a list, assigning them to a user, or changing their field values. To configure an autoresponder email to send as a completion action when the "Request a Demo" form has been submitted, you need to go to Marketing > Forms > Forms > Request a Demo > Completion Actions > Add New Action > Send Autoresponder Email, and select the email that you want to send.