I got an error from powershell:
This is because the Start-Process command has two parameter sets. Looking at the page linked, the parameters are listed in two groups. These groups can’t be mixed and matched. You have to pick one or the other. So if you want to do runas, you can’t use NoNewWindow.
This was a bit of a revelation to me, but I suppose it is one of those things that is so obvious nobody feels the need to point it out!