Hello,
This is already available by linking to a webpage where you've installed a coded sign up form, or linking to a landing page form you've created. Since coded sign up forms can be technically installed anywhere, and don't have a dedicated URL, it wouldn't currently be feasible to have a catch-all function in campaigns for subscribing.
See also:
Insert, style, or remove text links in an email
Clickable image links in an email
Insert and customize a button in an email
... View more
