View source worked for me, but there’s a couple of tricks to it I think…maybe.

First I made sure I was on the channel’s main page and not one of the channel’s videos. Then I did right-click “view source”. The source is all smashed together (unformatted) of course. It’s annoying to search and read like that, so I copy-pasted the source into my favorite text editor and was able to find “channelid” it that way.