Updated with link to full film. Exclusive: In the history of advertising, few if any taglines have rivaled the impact and enduring power of this one: “Because I’m worth it.”

That phrase, deceptively simple yet embedded with cultural meaning, became indelibly linked with L’Oréal Paris, the beauty brand which first embraced that line of copy more than 50 years ago. The words were the brainchild of Ilon Specht, who in the early 1970s worked as a young copywriter for the advertising firm McCann-Erickson (now McCann). Her story is told in The Final Copy of Ilon Specht, directed by two-time Oscar winner Ben Proudfoot.

The documentary began streaming for free today on the Ted YouTube channel, a day before International Women’s Day. The short film, a production of Traverse32, Proudfoot’s Breakwater Studios, and McCann, has been licensed by Ted for global distribution, “becoming the first film the platform has...