Campaign level confirmation email
Auxiliary emails are the emails sent when you have double opt-in turned on for a campaign.
Navigate to your Campaigns tab > Choose the campaign you want to work with, then go to your Emails tab and select Auxiliary Emails.
Feel free to modify this email as you see fit, but you will want to leave the following short code in the email so your new subscriber will be able to opt-in to your campaign.
I accidentally deleted my confirmation link
Not to worry! Simply copy and paste it from the warning above and all will be in excellent order.
Form level confirmation email
Navigate to Forms > Design > Confirmation Settings
Unless you select Never from the drop-down options, any subscriber that submits the form will be required to confirm their subscription via the confirmation URL in the confirmation email.
Save your settings.
Check out this doc if you would like to manage your double opt-in settings.