How can I force output to an external display during boot?

I need to have all output go to an external display. Including boot. The normal display isn’t working so I need to see the boot process and check for problems. It gives all indications of running and booting but no display,

It had been running fine with an adapter from the minidispiay port adapter to vga monitor.

yes, i’ve read, but not understood, all the articles.

+===================================

UPDATE:

++++====================================

Replacing the hard drive with one known to boot has a startup screen. With the previous drive in an external enclosure I installed the OS again and ran DISKUTIL to check for any file errors. Unfortunately it wasn’t just reinstalling the OS, it seems to have installed it over the prior. That’s just my guess since it boots now into a login screen asking for userid and password with no prompts or hints. I’ve tried combinations from the prior install and nothing unlocks the screen. There’s no wifi icon so I believe it’s not on.

None of the startup commands are visible on the external monitor. So my only option is to move it to another machine or to guess the user/password combination. So I’m asking you if you know or could guess a default user Apple has for these cases.

=====================================

UPDATE: 2/13 12:57

=====================================

It becomes more complicated. There may be a problem with the MacBook Pro’s disk subsystem. The new disk I just put in can’t be erased with DiskUtil because it complains there’s no firmware partition. I’ve used First Aid twice same as erasing twice. The original disk was erroring out as well. It’s more strange because DiskUtil says the drive is 750GB while the manufacturer label says 500gb.

Of course if I simply let the 500/750gb drive simply boot, it displays a generic prompt for USERNAME andf PASSWORD. Despite or in addition to Dan and my bewilderment, I never finished that install nor did I create any user. Which might be the reason. Of course I tried the one user always created, GUEST. No go.Nor any of those from my other installs.

All this because I pulled loose the display connector on the logic board. I’ll be working onit all this afternoon

DEVELOPMENT: Seems that having the boot drive be my SSD is causing the no firmware error. Also, I have some USB Flash drives setup as install drives. One must have gotten set as the startup drive. When it’s in, things boot and don’t if it isn’t.

This is just another day in Alice’s Rabbit Hole.

=====================================

Last Update

====================================

It’s been ridiculous, but it ended finally when I had the original drive back in the mac and then used an install USB to put the OS )High Sierra) on. Hopefully I’ll remember the steps it took. And remember that you activate the external as primary two ways” close the lid after turning it on.