We've enabled the functionality to allow you to include JSON-LD within a script tag in the head of HTML emails.Within the script tag, annotations can be added as JSON-LD. These email annotations will allow Gmail to display your message as a 'Top deal' in the Promotional Tab and highlight key information:

• Logo

• Deal Badge

• Discount Code

• Expiration Date

• Single Image Preview
  
  Read Gmail's offical documentation on adding email annotations for the Promotional Tab. You can also get more info in our guide to Promotional Emails.