This took a while to figure out, so hopefully it helps others -
The goal is generating a DoubleClick tag, that will take additional parameters and pass them to the destination landing page.
Login to your DoubleClick account, click on Campaigns, then select the Ad
Scroll down and open the Landing page URL suffix section
Enter the url key you'd like to populate, into the URL suffix box, in this format:
This will tell DoubleClick you want to pass "key" to the landing page, populating it with the value of key sent to the tag.
If you want to populate more than one key, use this format: