Add YouTube Videos to Playlist Directly From Search Results
Playlists are a great way to keep track of your YouTube videos. Typically you use the Add To button on the video page to add it to the list. YouTube search now allows you to do this by results.
When you search YouTube, hover your mouse over the video and you will see three vertical dots on the side. Click on the dots and you will see your playlists. Then just check the listings and this will add a video.
Quickly Add YouTube Search Results to Playlists | Google operating system