The default rights define in which cases an advertisement should be shown. If you would like to add an advertisement to a specific asset that differ from the default settings you can define an override.
A step by step guide helps you through the process. You can access this guide in the console by clicking on Ads --> Override and then Create override in the upper right corner.
Step 1: Select asset
Find the asset in the dropdown menu you want to add an override.
Click on select
Step 2: Ads settings
Define if the advertisement should be shown to all users or free users only.
Then activate pre-, mid- or postroll
Enter the VAST-tag
Step 3: Preview and create
Preview your advertisement set up and if all is correct click on create.