It sounds like you're referring to the 1987 comedy Il segreto del mio successo (original English title: The Secret of My Success ), starring Michael J. Fox.

You mentioned it’s streaming on — I can’t access real-time streaming availability, but here’s a quick recap of the story in case you need it:

Brantley Foster (Michael J. Fox) is a recent Kansas graduate who moves to New York City hoping to climb the corporate ladder. After struggling to find a job, he takes a mailroom position at a massive conglomerate run by his uncle, Howard Prescott. To get ahead, Brantley secretly uses an empty executive office and poses as a fast-rising hotshot named “Carlton Whitfield.” He juggles the mailroom job by day and “Carlton” by night, winning over colleagues and a romantic interest — his uncle’s mistress, Christy. The film mixes schemes, romance, and 80s corporate satire, ending with Brantley succeeding on his own terms.