Using HMACSHA1 handlebars to generate authentication tokens will allow you to use fields other than `email`, `campaignId` and `templateId` in your hosted unsubscribe page url. This is useful if you wanted to include userId as a merge parameter in your hosted unsubscribe url to take users to a page that is unique to him/her. In the url, the userId will be hashed.
Enable HMACSHA1 handlebars by going to Settings--> Project and adding an HMAC secret. This can be any combination of letters, numbers and symbols. It is similar to setting a password.
Now you will be able to include fields other than email, templateId or campaignId in your hostedUnsubscribeUrl on the Settings--> Project page.
If you need to grab an un-hashed userId from the hosted unsubscribe url, you will still need to add an HMAC secret and add HMAC Handlebars to part of your url.
This would allow you to add userId to the hosted unsubscribe url while still maintaining the original value of the userId for other purposes.