My screen is broke on my 2015 MacBook Pro 15 inch I want to know how can I start it up on another screen that I connect with the HDMI cord
Can I start my Computer and then switch with the HDMI to another screen.
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It can sometimes be tricky getting an external display to work properly if the built-in display is even partially functioning, but not viewable like in your case. You may need to use Clamshell Mode and an external keyboard & mouse (at least temporarily) until everything is configured correctly.
Connect your laptop to your external display. Then close the display on the laptop to trigger Clamshell Mode. If you get lucky you should see the login prompt on your external display. If you can log in, then make sure to configure the external display to be the main display.
If you don't have your login prompt (or you don't have an external keyboard & mouse to use Clamshell Mode), then try logging in blind by typing the first character of a user name, then press the "Return" key. Then type the password and press the "Return" key again to submit the password. Once you are logged in if you cannot navigate using the external display, then you can toggle mirroring by pressing Command + F1 (you may first need to press & hold the "fn" key at the front lower left corner of the keyboard before pressing Command + F1).