Not sure what you mean by "creating a workspace using cmd" but the code below runs designated workspace from commandline

import os

import subprocess

args = [r"C:\\Program Files\\FME\\fme.exe", "C:\\\\Users\\\\$(user)\\\\Desktop\\\\testpython\\\\Exercisela-Complete.fmw"]

subprocess.Popen(args)

U can set env variables in a command line and then call a workspace using these env variables using a EnvironmentVariableFetcher.