You need to pass each command line argument as a separate method argument like this:
Runtime.getRuntime().exec(new String[] {"netsh", "wlan", "start", "hostednetwork", "runas", "/noprofile", "/user:mymachine\\administrator"})
Otherwise, it seems like the command name is netsh wlan start hostednetwork runas /noprofile /user:mymachine\\administrator
.