The easiest way to do this is to use your existing footer menu to give visitors access to your referral link.
To do this, first be sure you’ve created an affiliate account. Then, go to Online Store > Navigation and locate the menu that appears in the footer. Click the “Edit menu” button and then the “Add new menu item” button. Give your link a name (something like “Designed by Out of the Sandbox” works well) and select “Web address” as the link type. In the second blank field that appears, paste the tracking URL from the affiliate site. Save your work.
You can also use the existing “Designed by Out of the Sandbox” link as a referral link. To do this, first make sure the link is activated by going to Sales Channels > Online Store > Themes > Customize theme (blue button) > Footer. Look for “Show theme designer credits” near the bottom and check it. Save your changes.
Next, go to Sales Channels > Online Store > Themes > “Actions” icon > Edit language. Then, search for "Designed by" and you should see this in the results that appear:
Designed by <a href="http://outofthesandbox.com/" target="_blank" title="(Theme Name) Theme by Out of the Sandbox">Out of the Sandbox</a>.
Simply replace the bold text with your unique tracking link and save.
For more tips on the affiliate program, see this blog post.