Add to Campaign with an email click?

Investigate using a Tracked Link in the emails. 

This is not necessary if you are sending the email from ONTRAPORT, as we automatically track all links in emails. You can select the email link click as a Trigger or Goal for ONTRAPORT emails. 

Using a Tracked Link works if the Contact is already in ONTRAPORT and is cookied, and the page they land on has the ONTRAPORT tracking script on it.

For new Contacts, they must still opt in to keep in compliance with the various spam rules. Use a simple opt in form on the page you are linking to.