You can use start-process
Start-Process -FilePath "cmd.exe" -ArgumentList '/c "java -jar fitnesse-standalone.jar -p 9090"'
The /c and everything following it is just part of the one set of parameters being passed to cmd. If you want powershell to wait for the the Java app to close, add the -wait parameter.
There's also no real need to use CMD at all in this case (since your giving it /c to exit immediately anyway), you can call java directly:
Start-Process -FilePath "java.exe" -ArgumentList '-jar fitnesse-standalone.jar -p 9090'
and it should be the same.
Note: you'll need Java in your path or the current working directory, and fitnesse-standalone.jar in the current directory for either of these to work.