Playlists are a great way to track your YouTube videos. Typically you use the “Add to” button on the video’s page to add it to a list. YouTube’s search now lets you do it from the results.
When you perform a search in YouTube, hover over a video and you’ll see three vertical dots on the side. Click on the dots and you’ll see your playlists. Then just check the lists and that will add the video.
Quickly Add YouTube Search Results to Playlists | Google Operating System