If you need to insert a video from YouTube direct into Powerpoint (without downloading it), you can use the following method;

In Powerpoint go to Insert > Video > Online Video

Upload YouTube Video to PowerPoint


From there you can easily search your YouTube Video and insert it into your presentation.

Insert YouTube video


But always keep in mind, that you only can watch the video, if you have an internet connection during your presentation. Otherwise, you need to download the video.