RFC 8058 header for one click Unsubscribe

Ok I got this sorted. 

Here is an image of what I used in order to satisfy the RFC 8058 requirement:

Hi

I asked this same question once to customer.io support. Maybe this answer will help you as well. 
 

in short: you don‘t need to manually add this header. One click Unsubscribe will work automatically unless you send a test email. 
 

André

Jumping right into your question here, based on the wording in your initial email, it sounds as if you may have used the 'send test' button to send the sample email to this testing website (aboutmy.email), rather than sending that email through a real campaign or broadcast. Is that right?

 
If so, this is actually the reason why those headers would have not been included. Test emails (using that 'send test' feature) are not expected to contain personalized unsubscribe links, meaning they also won't attach the required headers for the unsubscribe link to work properly. This is outlined further in this help document if you'd like to take a look!
 
As long as you are using the Liquid tags for your unsubscribe links (as it sounds like you are), no further action is required on your side, and the headers will be included automatically on all real sends. If you want to use a service like aboutmy.email, you would likely need to add the email you're sending it to as a user on your workspace, followed by creating a broadcast targeted to just that one user, and sending it as if it were a real newsletter. Once you do that, you should see more expected results!