The pulp doco says
Command+B : Run your game in the Web Player (Using Command-B in the Web Player will switch back to the Editor.)
Ctrl-B in editer goes to web player.
Ctrl-B in the web player doesn't work.
I've tested in 2 browsers, and have had 2 other people also test.
Is this just a windows thing?
Nope, not just Windows. Command-B from player to editor doesn't seem to work on Mac either. Thanks for the report!
In some browsers, for JavaScript to be able to handle any input events (or play audio) in a pop-up window the user must first directly interact with the page (ie click somewhere). I just tested in Safari and Chrome on macOS and it works (after you click somewhere in the player window).
Looks like Firefox was having some trouble targeting the editor window that opened the player window. The latter has been fixed and will be available shortly.
Hey sorry if my bug report was a bit terse.
I can understand that you may need to focus on a window for it to hear your inputs.
But I just tried again in Edge and Chrome and had no luck. 
Others in discord reported the same.
Please try again now, I don’t think the change had gone live when you tested it. 