Make an Existing Form Confirm Opt-In:
Manage Forms > My Forms
Confirm Subscribers (Opt-in)
A confirmed (Opt-in) subscriber is one who has not only signed up for your list but also replied to a message which validates their subscription.
Step 1.
Locate the existing Form you wish to confirm Opt-In and click on the 'Edit' icon.

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Step 2.
Make sure that you enable the 'Opt In Confirmation'.
Confirm Opt In: A confirmed subscriber is one who has accepted to receive campaigns after clicking a URL to validate their subscription. It is a good idea to ensure that you have as many confirmed subscribers as possible to protect you from any spam complaints.

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Step 3.
Click 'Save and Continue'