Can I use Karabiner to disable built in keyboard during start up The D key on my built-in keyboard (MacBook Pro) is stuck and it keeps looping into self diagnostic when I boot up. I would like to disable the keyboard during computer startup and go straight to the external keyboard but have not found a way to do this.

If I install Karabiner on my MacBook Pro and disable in Karabiner settings the built in keyboard, after I turn off the MacBook Pro and start it up again with the external keyboard plugged in, would the built in keyboard function during the start up or would it be disabled? 

If not is there another way to disable the builtin keyboard during startup?
No, Karabiner won't help here, because it first has to be started to disable the internal keyboard.

You have not elaborated on why the key is stuck, but maybe you can fix it? 
https://www.ifixit.com/Guide/How+To+Fix+Sticky+Keyboard+Keys+on+a+MacBook/37709 shows you how to remove a key cover from the keyboard. There are also videos out there. 
Also Apple has a guide you might want to check: https://support.apple.com/en-us/HT205662