We moved from Geshi to CodeMirror because CodeMirror is realtime (so you can have coloring as you type) and the license permits us bundle in Tiki, instead of forcing end user to install.
Unfortunately, PowerShell is not by default in CM: http://codemirror.net/mode/index.html
I just tried this on 9.x:
I added the content of https://github.com/VapidWorx/powershell-cm-mode to lib/codemirror/mode/powershell
Seems to work. Can you try different syntax? Password is indicated on demo.tiki.org
If it works, next steps would be to get the folks behind https://github.com/VapidWorx/powershell-cm-mode to share the code to https://github.com/marijnh/CodeMirror/tree/master/mode and, then, it will be by default in Tiki.
In the mean time, you can fiddle with your local copy.