How to add an Unsubscribe link to an HTML email in Campaign Builder?

The situation is this. A client has sent us an HTML edited newsletter that we want to send out in Infusionsoft. However, they have included Mailchimp tags for the opt-out at the bottom of the newsletter.

I would like to simply replace the link with a link to my Infusionsoft opt-out page, however I can find no such link or advice on how to find it in Infusionsoft’s help centre.

The closest they have is information about custom opt-out pages for Legacy users, but this method states it is not compatible with Campaign Builder and does not provide a URL I can add to the HTML of my client’s newsletter. You can see this help section here:

I had considered simply clicking the URL of an unsubscribe button in a previous campaign and adding that to the bottom of my HTML newsletter, but these URLs are campaign specific, so I can’t be sure that they will work as expected when I use them in this new campaign with an HTML newsletter.

There must be an easier way to do this.

Any help would be much appreciated!