While drafting there's a built-in chat box, so I can't hit Shift+Enter to bring up all chat. I've seen people still use all chat somehow though. How does one use all chat during drafting?
I don't think there's a way to do it in the GUI. As you've seen it is possible, but you have to use the console.
First, you have to have the console enabled. Then just hit `, or whatever the console is set as and type