In desktop select display settings (right click , display settings), now if you wanna see both as duplicate, on multiple displays select Duplicate, if you wanna see as extended select extended.
Running windows 10 but do not get that option. I know what you mean when you have a laptop connected to a PC which already has a screen there is usually an option to see both screens ?
I am just connecting my desktop PC (with HP screen) with my LG TV via HDMI. All works perfectly except my PC screen goes black and then sleeps so i have to strain my eyes to try and read the TV screen. My reference to laptop was purely to say I know when connecting my laptop to my PC I do have an option for multiple screens. When I right click on my HP screen I do not get those options for multiple screens.