2013 iMac use external sceen only

[Rumpot]

Is there a way to do this in ElCap? I am getting a new 21:9 monitor to use for both a PC and my iMac and would like to turn off the iMac screen.

I found this but it is old

https://forums.macrumors.com/thread...-screen-but-leave-secondary-screen-on.838040/