The best way to have a thank you email sent after the form on the "Contact Us" page is submitted is to send it using the form's completion actions. 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. You can also use completion actions to send autoresponder emails, which are emails that are automatically sent to a prospect after they complete an activity. Autoresponder emails can be used to thank your prospects, confirm their actions, or provide them with additional information or content. To send a thank you email using the form's completion actions, you need to go to Marketing > Forms > Forms > Contact Us > Completion Actions > Add New Action > Send Autoresponder Email, and select the email that you want to send.