Steam Broadcasting is a neat feature I've been using with a friend to watch each other play some games remotely. There is a chat room in the broadcast where the person casting can see people's chat messages without even leaving the game window. For other participants, it also says that person is in the chat room, but neither of us have been able to figure out how to participate in that chat as the broadcaster.
How can the broadcaster type into his own broadcast chat?
Best Answer
It's really simple: Open your Steam Overlay, and in the "Broadcast" section, select "Open Chat".
Nobody will notice you opening the Steam overlay in the broadcast.
After that, simply
alt-tab out oropen the overlay to talk into chat.For now, the chat window is limited to the steam overlay. It does not open or appear if someone talks into it. (Which is fair - Who wants lots of notifications?)
You can talk in chat by simply (if you're not the broadcaster), by typing in the chat window on the right of the broadcast. It is in the same location when viewing both in the overlay, Steam Client or web browser.
Otherwise, type into the chat window in the Steam overlay.
If you do not see a broadcast tab, it's probabally because you did not start one, or nobody is watching you. If you've started broadcasting while the overlay is open (and the tab did not open), simply close the overlay and reopen it.
A side note:
If you're looking for a way to chat outside your game (alt-tabbing), the only way is to watch your own Stream. (the "Watch game" button appears on your profile, even for the broadcaster).
Though be warned - This method uses double the bandwidth. Not a good idea if you have an internet cap.
Depending on the broadcaster's broadcast privacy settings, he/she'll still have to manually accept your request to watch. (For my case, it's public; anyone can join and watch without permission.)
Note that I'm actually viewing this page logged in as myself, as the "Edit Profile" button is there.