The short answer
Every email sent through a campaign automatically includes a List-Unsubscribe header. This header enables one-click unsubscribe in email clients that support it (such as Gmail). You do not need to do anything to enable the header; it is added to every outgoing campaign email.
If you want a visible unsubscribe link inside the email body, you need to place the {unsubscribeLink} merge tag in your email design. The blank starting template includes this tag by default, but if you have removed it or started from a custom template you will need to add it back.
The default template
When you create a new email campaign, the starting template includes a text block at the bottom that contains {unsubscribeLink}. At send time that tag is replaced with an anchor element labeled "Unsubscribe" pointing to a unique unsubscribe URL for each recipient. You can move, style, or re-label that block inside the email composer, but you should not remove it.
Inserting the merge tag manually
If you are working with a custom template and the tag is not present:
Open the email composer for your campaign.
Add a text block where you want the link to appear (typically the footer).
Type or paste
{unsubscribeLink}into that block.The composer's merge field picker also lists "Unsubscribe Link" under the available fields if you prefer to insert it through the interface.
At send time {unsubscribeLink} is replaced with a working anchor: <a href="...">Unsubscribe</a>. The URL is unique per recipient and per campaign.
What happens when someone unsubscribes
When a recipient clicks the unsubscribe link, they are taken to an unsubscribe confirmation page. After confirming, their email address is marked with a removal reason in your contacts. Future campaign sends automatically skip any address that has been marked unsubscribed, bounced, or flagged as a spam complaint. This suppression is applied per campaign send; you do not need to manage it manually.
Checking before you send
Before sending or scheduling, open the preview or send a test email to yourself. Verify that the unsubscribe link appears in the email body and that clicking it loads the confirmation page. The List-Unsubscribe header is added automatically and does not appear in the visual preview.