My dictionary says "forward". Remarkable is the existence of a complete different word "foreword", which a author writes at the beginning of a book.
As a not native English speaker I'm confusued about a similar problem: Am I right - "You walk forward or you walk backwards, you do a backward walk" What about the 's'? My theory: "forward" always without 's', but "backward" as an adjective and "backwards" as an adverb. Any comments on that?