1. Click Subscribers.
2. Click my lists.
3. Select the checkbox next to the list for which to define a double opt-in status.
4. Click Properties
5. Select the Require subscribers to provide confirmation of valid email address(double opt-in) checkbox.
6. Click Select and choose the email to send as the double opt-in send. The substitution string to insert the link in your email is %%double_opt_in_url%%.
7. Click Save.