Hitchcock's cameo in Lifeboat (1944) occurs about 25 minutes into the film, where he appears in a newspaper advertisement for the "Reduco Obesity Slayer" weight loss system, manufactured by fictional The Reduco Corporation.

The advertisement appears to read:

The New Sensational Reduco OBESITY SLAYER In Just Four Months YOU TOO Can be Slender ! The Well Known Director ALFRED HITCHCOCK SAYS "In just Four Months I lost 72 pounds due to the splendid new Reduco System!" THE Reduco CORPORATION NEW YORK CITY BOSTON

The confined setting of the film limited Hitchcock's options for his now obligatory cameo. After toying with the idea of appearing as a corpse floating past the boat, he decided instead to make use of 1943 photographs showing his dramatic weight loss from dieting.

According to an in-depth article on the production of Rope (1948) by American Cinematographer editor George E. Turner, the red neon sign which forms his cameo in that film was intended to be the a follow-on gag and was the logo of the Reduco Corporation, who are based in New York and Boston:

As the sky darkens, lights begin appearing in the windows of the buildings and neon signs flash on. One of these signs depicts before‑and‑after figures advertising a product called Reduco. The model for this was the director, whose portly figure is well known, making the token personal appearance that had been a tradition in his films since The Lodger, made in 1926. He had used the Reduco gag as a newspaper ad in Lifeboat (1943), another one‑set show in which he could not logically do a walk‑on.[1]

This was also confirmed by a brief comment made by Hitchcock in his 1948 article, "My Most Exciting Picture":

It's traditional, with me at least, that I appear fleetingly in every one of my pictures. But Rope, with a cast of only nine people who never leave the apartment, looked like the end of the Hitchcock tradition. There was just no way that I could get into the act. Then someone came up with a solution. The result? The Hitchcock countenance will appear in a neon "Reduco" sign on the side of a miniature building!

Film Frames

Selection of film frames: Alfred Hitchcock in Lifeboat (1944) (click image to view larger version)...

