Sometimes it can be extremely difficult to get video on the external screen when the built-in is not working.
Command + F1 should do it (perhaps fn then add Command + F1 ?), but I have found that this and the screen brightness may not work if sitting at the login screen (especially if Filevault is enabled).
The most reliable method is to connect the power adapter, external display, and a USB Keyboard & Mouse. Yes, you will need to use an adapter to get them all connected since there are only two USB-C ports. With all of these items connected, then you can close the Display Clamshell lid and video should go directly to the external display. From there you should be able to configure the Display System Settings to make the external display the primary display so Clamshell Mode is not required.
There is a possibility you may be able to get video onto the built-in display by closing the Display Clamshell lid, then very slowly & gently lift it open just about a quarter inch. Peek in to see if there is video on the built-in display....if there is you may be able to lift it open a bit more, but no more than about 45º since if the issue is with a broken cable (very common issue with the 2016 & 2017 models), this is usually the point where the video/backlight may go out. It should be enough for you to use an external keyboard & mouse to modify the Display System Settings to show video onto the external display.
If you normally have a list of login users shown, then you can type the first character of a user's name & press the "Return" key. Then enter the password blindly making sure to press the "Return" key to submit the password. If multiple users start with the same character, then you may need to try passwords for each other account since I don't know which one would be accessed using this method. If Filevault is enabled, then you will need to give it a minute to finish booting before you may see anything on the external display. If you entered the password incorrectly, then just type it again after waiting a few seconds.
If the display backlight is bad, then try using the laptop in a completely dark room to see if you can make out a faint image on the screen to navigate in order to log in & configure the Display System Settings. Also try using the Brightness button to increase brightness just in case it works.
