1. Within you TTD ad group, find and select the Creatives rail.
2. Within the Creatives rail, click the Browse button to add new creatives to the existing Ad Group.
3. Use the search bar to find the creative you'd like added to your Ad Group. Check the checkmark to select the specific creative or creatives (multi-select). Click Add.
If you'd like to apply a custom headline/description to your new creative, don't forget to reach out to your Sharethrough rep to implement!
If you'd like to target Sharethrough supply against a specific Ad Group, please refer to here.
For complete details on setting up a deal ID in TTD, please refer to here.