Email Markup => Go-To Actions => ViewAction
https://developers.google.com/gmail/markup/reference/go-to-action#view_action
Example from ViewAction web site
<script type="application/ld+json">
{
"@context": "http://schema.org",
"@type": "EmailMessage",
"potentialAction": {
"@type": "ViewAction",
"target": "https://watch-movies.com/watch?movieId=abc123",
"name": "Watch movie"
},
"description": "Watch the 'Avengers' movie online"
}
</script>