Where is the drawing canvas made in the JS file?

[William]

I'm looking to add an ID to the drawing canvas so that I can save its state and send it to my server. However, I'm not sure where the canvas is actually created so I can't add an ID. Can anyone provide any assistance?

I think this is where the container for the two canvases are, but I can't find where they themselves are created.

I'm not sure what this is, I just searched for 'transparent' as that's what colour the canvas was. I tried to add an ID to it but it doesn't show up.