The ID returns once your upload is completed.
Edit: I don't know the answer but I assume from all the APIs I worked with in the past, but if you get the URL before the upload is completed then yeah it's pretty easy to "fake".
Edit: I don't know the answer but I assume from all the APIs I worked with in the past, but if you get the URL before the upload is completed then yeah it's pretty easy to "fake".
My guess is that you can start an upload, get the ID, and then stream unimportant and non-comitall video headers and chunks (e.g. subtitle tracks or other metadata) slowly enough to keep the server from closing the connection on you. Then you can just record your video and start uploading actual video chunks once ready.
Or maybe YouTube doesn't have a huge timeout for starting your upload, so you don't even have to do all of that.
So it seems you can get the URL immediately... maybe then stop the upload mess with the video and then resume the upload... but I assume the resume feature takes a hash or something to make sure it's the same file..