So here is the link. You download the zip file from ckEditor
https://ckeditor.com/cke4/addon/youtube
You extract that zip file in
PF.Site/Apps/core-CKEditor/assets/ckeditor/plugins/
There are three folders (ckeditor, ckeditor_basic and ckeditor_full) but I put it in the ckeditor folder as shown in the above URL.
Then you go to
PF.Site/Apps/core-CK-Editor/assets/ckeditor/config.js
and add at the end of line 39 youtube like this:
config.extraPlugins = 'youtube';
Refresh the cache and your done.
If you already have added extra plugins like I did you have to add it to the current line. Remember you will need to corresponding plugin folders. My extra plugins that I already had installed are justify, font, colorbutton, panelbutton and required floatpanel and the newest one youtube. These were all individual folders download from ckeditor site and put in PF.Site/Apps/core-CKEditor/assets/ckeditor/plugins/.
config.extraPlugins = 'justify,font,colorbutton,panelbutton,floatpanel,youtube';
Here is an image of a successful install of the config.js file.