How to use an external monitor on macbook pro with broken screen?

So, my screen is completely broken and I removed if from the macbook pro. Now I want to use it with external monitor but cannot really do that. Well it worked few times but could't understand how that happened. Now I'm stuck with it, nothing works, I try in safe mode, I try with Cmd+f1 etc...I'm sure there should be an easy combination or option to do that, so help appreciated!

Try to enter the settings, then the monitors and click on the location tab .. There you can choose the priority between the displays.