In American and Canadian publications, kickoff is a noun and an adjective, and kick off is its corresponding phrasal verb. For instance, it is one word in “the kickoff time is noon” and “you missed the kickoff” because it is an adjective in the first example and a noun in the second. And it is two words in “the kicker is ready to kick off” because here it functions as a verb.
The same distinction applies outside North America, but the noun/adjective is usually hyphenated—kick-off.
In the kickoff to a thriller series for young adults, Julie Cross introduces time traveler Jackson Meyer. [Los Angeles Times]
Things kick off with 2001′s MGS2, less a sequel than a postmodern meta-commentary on the game that preceded it. [Guardian]
[I]t represents the kick-off point for a much-improved business year that will see several other new and facelifted Peugeot models arrive. [Taranaki Daily News]
City officials said as many as 10,000 people attended the event, which was meant to kick off a series of similar demonstrations across the country. [New York Times]