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

Saw a piece
actions-go-to-action.png

To do is resource

The question is how this can be done directly in gmail in the web interface. Couldn't figure out where to insert the code.
July 2nd 19 at 17:50
1 answer
July 2nd 19 at 17:52
Well, there are written all

<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>

Create an html email, paste it into code and send

Well, How to send HTML email?

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