<map lat="57.441544357848876" lng="10.524135609714538" zoom="12" view="0" float="right" />Frederikshavn is a city in Denmark.
Maybe there are different kinds of ferries, but when I hitchhiked this route September 2014, I had to pay 6 EUR even though I hitched a ride. Anyway, if you go just as a passenger on foot it is 18 EUR.
As of Feb 17, 2017, it was impossible for us to hitchhike the ferry to Oslo. It's cheaper to board the ferry as an additional passenger on a car than as a single on-foot passenger, so we tried to thumb before the car check-in in the harbor, but security kicked us out saying we couldn't hitchhike and threatening to call the police. We moved just out of the harbor and the guy came back threatening us again, and said he would pull us out of any car we could've gotten in to go through the check-in. So we had to buy full-fare tickets. If you do this, do it at least two hours before departure, cause the basic fare got sold out and only the more expensive flexi fare was available. Long story short, we ended up paying 24 euros each instead of 6 (that's what you'd pay as an additional passenger). I guess the only possible way is to thumb way out of the harbor and well out of sight of check-in security.