The keys to the farmhouse can be found on the dashboard of a nearby truck. The outbuilding nearby contains a dead body being ravaged by a wolf and a workbench. The body can be looted for the key to a lockbox inside the farm.
As well as a lockbox in the office of the farmhouse, there is also a safety deposit key (#15) located in the urn in the bedroom, but this cannot be obtained until Grey Mother asks Will to find it as part of the main Episode 1 questline.
In sandbox mode, they key to the farmhouse may be found at various locations around the property, including on the dashboard of the pickup truck, or on a frozen corpse behind the farmhouse.