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.

By default with X. But Steam has to be running for that to work. If the controls are in gamepad mode (you can switch by long-pressing the start-button, the one with the three lines) you can use Steam+X.

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.