How to play as a guest/anonymously while logged in?

If you're not logged in and hit the Play button you're given an option to play as a guest in unrated games with a randomly generated username. However I sometimes see players writing chat messages in such games, but my chat window is grayed out with a note that I have to register and login to send any messages back. Also sometimes players are able to use premoves, which a regular guest can't enable. So how could this be done? I haven't seen any way to play as a guest when I'm actually signed in. Thanks. 

No, you can’t play as guest while login. 
“Also sometimes players are able to use premoves, which a regular guest can't enable. So how could this be done?” —— It is possible as guest to do that.

“However I sometimes see players writing chat messages in such games,” — In watch, guests can predict moves, and they appear in the top people who predict moves in the chat if that was what you mean.

Turns out you can, just figured it out. You have to start a game as a guest then login in a separate tab. Then refresh the game tab and you'll still be playing as a guest but will now be able to use the chat. And yes thanks, I see that you can enable premove as even as a guest (by clicking on the little gear icon next to the board). 

Now that I think about it the people that used the chat window only sent emojis and never wrote anything. Is there a way of sending emojis without doing the workaround I mentioned above? 

Yes, I forgot to mention that when playing as guest, they can say simple words like : “Good game” “Good move” “Hello” “Gotta go” , this is an option beside your guest username that looks like bubble text with three full stops. Click on it, and you will find some words you can use it, and emojis. But it’s my first time to heard about what you said in paragraph 1.

He found out that it is possible