This particular Fallout 4 mission is a 2 for one special. You’ve been asked to clear out the National Guard Training Facility by Greentop Nursery. While there you’ll also come across a distress beacon that starts “Lost Patrol” to investigate the nearby satellite array.
There’s Feral Ghouls all over the place, and they suck, so bring Moltovs. You have to clear them out anyway to complete the Militia missions.
In a corner room on the main floor there’s a dead Brotherhood of Steel woman with the emergency beacon. Grab her stuff and listen to the holotape to get the basic message.
The next area over is a locked door with an Expert terminal next to it. If you don’t have Expert hacking, don’t sweat it, there’s an app for that.
Go upstairs and take a sharp left around the collapsed floor. There’s a skeleton in front of a desk. In that room search the desk and find the password. Bingo. Drop down, enter the terminal and you are in. That’s it for this quick and dirty tip!