You’ve talked to Reed Tolbam, and you want to get into his locked office. There’s a couple ways to go about this, so how to get Reed’s key is simple.
Either pickpocket the key from Reed, or kill him for it.
Pickpocketing his key is the hardest option. There are two guards in the room with him at all times, so you’ll need high stealth to get away with the theft unpunished.
Naturally, there is an easier route: kill Reed. If you wanna complete Comes Now the Power to perfection, I recommend siding with Adelaide after you divert the power to Edgewater first. At this junction she will ask you to either peacefully or violently dispose of Reed, and at this point you can kill him without any major setbacks. This will piss of Parvati, but it isn’t difficult to convince her to remain on uour crew. Just go to his office, and shoot him in the face. You’ll need to fight him and his guards, but it isn’t a difficult encounter if you are around level 5 or so. Just kill everyone in the room, and loot the key off of Reed’s still warm corpse.
You can now unlock his room, which has a container with some items, a terminal with some additional lore, and his bowler hat on a self.
That’s how to get Reed’s key: it requires some dubious ethics all around, but in the end you’ll walk away with a fancy hat.