In order for the purchaser to see the link to the Guest Form, you will need to insert that link into your Thank You Message.
While editing the Thank You Message, click your cursor where you would like to place the link and then click the "+BTN" button. This will launch a dialog box. Select the dropdown at the top of the dialog and scroll down and select "Event RSVP." Next, in the second dropdown, select the name of the Event.
Once you select the correct form, the Button Text and Button URL fields will be automatically populated. You can change the Button Text but leave the URL. Then you can style the button and Preview it before you click insert. Once inserted this button will act as the link to allow the purchaser to open the Guest Survey.