Back in August, I noticed that YouTube added a menu next to search results, so you can add them to playlists. The menu is now even more useful because it has a search box and you can find a playlist faster.
Click the 3-dot icon, type a few characters from the playlist's name and pick the right playlist. You can even use keyboard arrows and press Enter. When you select a playlist, YouTube moves it to the top of the list.
If you want to create a playlist, you can type the name in the search box and select "Create new". Make sure to change playlist privacy setting to "private" or "unlisted" if you don't want to create a public playlist.