How to insert buttons in email subject line in gmail web interface?

Question

gmail

I saw such a trick

To do this, there is a resource The
question is how can this be done in gmail directly in the web interface. I can't figure out where to put this code.

Answer 1

Well, it's all there

<div itemscope itemtype="http://schema.org/Event">
  <meta itemprop="name" content="John's Birthday Party"/>
  ... information about the event ...
  <div itemprop="potentialAction" itemscope itemtype="http://schema.org/RsvpAction">
    <meta itemprop="target" content="https://events-organizer.com/rsvp?eventId=123"/>
  </div>
</div>

You create an html letter, paste it into the code and send it.
Well, how to send an HTML letter?