How does Basecamp add "View on Basecamp" links to Gmail?

Question

I just got an email from Basecamp and it included a link to "View on Basecamp" which I could click from my Gmail inbox.

How do they do that? I looked at the raw message in Gmail but I couldn't see any headers that included the appropriate link - it only shows up in the actual message contents. Is this a Gmail/Basecamp specific partnership or is it something I can do when sending emails through my app too?

Answer 1

These are Gmail Actions, declared using JSON-LD or Microdata embedded in the email.