Right click on an object or image and select EDIT - or click on the 'pencil' in the menu below, it should turn blue, and select the object you want to edit. 

The edit menu will open at the left of your screen

scroll down in the edit menu to 'Add a link'

Select 'Web link'

Go to the video on YouTube and copy the last part (the unique code) of your YouTube video URL,
e.g. https://www.youtube.com/watch?v=TheUniqueCode

Enter a new URL that embeds the video and finish with the unique code https://www.youtube.com/embed/TheUniqueCode

At 'Open Link In', select 'Embedded overlay'

Click on SAVE

Close the edit window by clicking on the blue pencil icon at the bottom of your screen

When you click on the object or image you will get a warning message on screen asking you if you want to open the link

it should open in the left corner of your screen

NOTE: Not all YouTube videos are set/enabled to be embedded so it might not work