The Urban Myth is just what the name implies, a widely believed perception that is not true. Urban means a large population or dense population, essentially implying a majority of people. Myth means false. There is a nuance in the term legend. A legend has a historical basis but is not provable. For instance, Robin Hood is referred to as a legend. The character or legend is ostensibly based on a person or persons that may have existed in fact.
A myth might be a dragon, obviously a creature that does not exist. A dragon is impossible but a bandit that steals from the rich and gives to the poor is possible.
The bar for falsehood is set higher for the term myth than for a legend. If we want to emphasize that a widely believed narrative is false we would use the term myth to describe it.