Open your video in YouTube and click on the Share option.
Select the entire code and click on the Copy button.
Choose the Insert tab click the Video button and select the Online Video option.
Select the From a Video Embed Code option by clicking in the box and paste the code (ctrl+v).
Then click in the embed code and position the cursor using the keyboard arrows at the end of the video path (starting with https://.., make sure though that you stay within the quotes. Then type ?autoplay=1, press enter or click the arrow next to the box.
<iframe width="560" height="315" src="https://www.youtube.com/embed/NpqmGx7meQw?autoplay=1" frameborder="0" allow="autoplay; encrypted-media" allowfullscreen></iframe>
Start the presentation, the video should start automatically.