Upload a video, that's 3 seconds or less long, as unlisted.
Then go to your InVideo Programming page.
CLICK add a branding video.
SELECT a video that's under 3 seconds long from those presented.
APPLY to videos uploaded after a specific date, OR to all videos.
YouTube will then post a message saying: "Applying bumper. This may take some time to apply to all videos."
Then as it starts working another message pops up:
"Branding intros are currently being processed on 2 videos. Please wait for processing to complete before selecting a different branding intro."
Here you can see it processing on my two most recent video uploads.
Here's my example video:
Note: Based on the htime it takes to process, it looks like the intro is actually edited in to the source video, and then re-encoded for web display, vs. being implemented in a playlist mode where intros could be swapped out more quickly.
Learn more at Boost your channel’s recognition using new intro videos.