Yup, you earn it in a sidequest. I think you gotta talk to that guy at that small camp in the Estersand, that gives you the small cactaur hunt, he tells you about his wife is at that river camp. Then you talk to his wife and she tells you about this wounded explorer and you go on a quest to help heal the person. The end result is getting the key.

Though you might be able to start it just by talking to his wife first instead of him.