I use a laptop and a 32” monitor. The way I did it was in power settings( I think) it will ask what you want to happen when u close the laptop cover, lid, screen whatever it is called. I set mine to do nothing when lid is closed. Start up procedure for me is open, hit power button close lid and it powers up my monitor and I am using only my monitor from then on. Hope this helps.
It looks like WoWP is coded to start on your primary monitor. You will need to go into display settings and set your external monitor as your primary monitor.