To include user-generated content in an ad, like videos uploaded by YouTube creators, the advertiser must get explicit permission from the owner of that content. This rule applies to all ad formats except for homepage mastheads. User-generated content is prohibited in homepage mastheads even if the creator has given permission. Advertisers can feature creator videos in their brand channel through existing channel functions (like creating Playlists of the videos or adding them as Favorites).
Some limited use of third-party social media APIs or features is allowed when approved by YouTube. Learn more about third-party apps.