Word for a temporary project name

Solution 1:

Working title or Production title

According to Wikipedia:

A working title, sometimes called a production title, is the temporary title of a product or project used during its development, usually used in filmmaking, television production, novel, video game development, or music album.

Working titles are used primarily for two reasons — the first being that an official title has not yet been decided upon, with the working title being used purely for the identification purposes, and the second being a ruse to intentionally disguise the real nature of a project.