"Go to shop"
button on page. If user is logged in, he will see direct link to shop (with your tracking link). If user is not logged, he will be redirected to register page."site.com/wp-login.php"
Also, we recommend to install Buddypress on site for User profiles. Then, you can use here link "site.com/register/"
which goes to register page of Buddypress.Theme option
-> User option
.