For advanced developers, you can use Velocity scripts in your emails. Here's how to do it.
Go to Marketing Activities.
Find and select any program (Event, Default, or Engagement, etc).
Under the My Tokens tab, drag in an Email Script token.
Name your email script token and click to edit its content.
Use the tree on the right to drag in Person, Opportunity, or Custom Object tokens.
Notice the token becomes checked/active after you drag it into the script editor.
Write your script in Velocity. Here are some useful resources:
Once your script is completed, click Save.
Click Save one more time.
Now you can use this token in your emails. It will run the script every time an email is sent.