Byobu with tmux, how to use F-Keys

I'm accessing a linux box with Byobu enabled (with tmux) and I don't seem to be able to use some F-Keys without being binded to Byobu. For example using F6 to sort on htop. I followed this advice , but it doesn't seem to work for me. I press Shift-F2 and then F6 but nothing happens. I'm connecting through PuTTY.