Targeting in Google's Ad Manager 360 is achieved by using the custom targeting feature. First you'll need to create a targeting key/value pair in GAM 360 for custom targeting and add it to your GAM line item, then set up a persona in ONEcount's Target Engine.
Create Key / Value Pair
For information on how to apply a Key-value pair to a line item or creative, visit this link:
https://support.google.com/admanager/answer/177381?hl=en
Troubleshooting
If your targets are not working as expected, check out this Troubleshooting Document.
Set up a Segment and Target Action in ONEcount
Create an Ad Targeting Action using the defined key-value pair .
Create your segment in the Target Enigine and attach the Ad Targeting Action to it.
That's it! The line item with the defined key/value pair will be targeted to the audience segment you selected.