As long as Steam is running, Steam + X should bring the keyboard up.

Alternatively, you can add a shortcut to the taskbar that opens it.

Steam+X as people have already said. I just wanted to add that the Steam keyboard is missing a lot of buttons that are very necessary when trying to use command line apps, for example. I installed another keyboard from the Discovery store to solve that issue and that’s working much better for me.

Steam+X should work in both game and desktop mode.