SparkPost offers substitution variables which allow users to pull in personalized campaign and user data. By using combinations of these tags we can generate and pass unique identifiers (UUID) into the key parameter without the need for any javascript. For example:
{{email_id}}  pulls in your campaign’s unique email ID
{{message_id}}  pulls in your unique message ID
{{timestamp}}  pulls in the date and time in Unix format

By combining these tags we can successfully create a dynamic UUID and our key parameter would therefore look something like this:
key={{email_id}}|{{message_id}}|{{timestamp}}

NOTE

  • the same UUID must always be passed into the key parameter of both the anchor and image tag
  • if running multiple HTML Ad units within the same email, variables can be rearranged or added to ensure the UUIDs of each unit are unique 
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