If you activate the double opt-in procedure in the email settings of your form, an opt-in link will automatically be sent in the verification email. You can also add the opt-in link manually by copying the dynamic element opt_in_link from the left and add it to your email text. According to the same principle, you can also add an opt-out link to the confirmation email. Depending on the current status of a form entry, you will also find the links under Forms > Settings to copy and resend them when clicking on a particular form entry.