Insert Unsubscribe into Gmail Email

I have spent the majority of my day researching on Google to try and find out why all of my emails are going to people’s spam folders. I’ve made the changes recommended, except one:

I can’t figure out how to insert an unsubscribe option into my emails. I know it goes in the header, but gmail doesn’t seem to have editable header text for emails.

Please, does anyone have a tip on how to do this?