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 Hyperlink Icon. 

This will launch a dialog box. Click on the Button Type dropdown towards the bottom dialog and select Event RSVP from the picklist.

In the next dropdown titled Button List you will need to select the option with the name that matches your event. The URL for that Guest Form will then be automatically populated into the URL field and once you click OK it will be inserted into your Thank You Message.