How to add link in subject of email (gmail)

I'm glad you asked! This is an awesome new feature and standard Google is supporting, part of the Schema.org initiative.

Copying their example here which allows customers to review something directly from the email! This block would be copied directly into your mail template.

<script type="application/ld+json">{  "@context":               "http://schema.org",  "@type":                  "Restaurant",  "name":                   "Google Cafe",  "action": {    "@type":                "ReviewAction",    "review": {      "@type":              "Review",      "reviewRating": {        "@type":            "Rating",        "bestRating":       "5",        "worstRating":      "1"      }    },    "handler": {      "@type":              "HttpActionHandler",      "url":                "{{ review_url }}",      "encoding": {        "url":              "http://schema.org/Encoding/UrlEncoded"      },      "method":             "http://schema.org/HttpRequestMethod/POST",      "requiredProperty": {        "@type":            "Property",        "name":             "review.reviewRating.ratingValue"      },      "optionalProperty": {        "@type":            "Property",        "name":             "review.reviewBody"      }    }  }}</script>

Documentation on how to activate these features can be found here: https://developers.google.com/gmail/actions/