the magnetic moment of a particles is given by, m=kS, where k is a constant the gyromagnetic ratio

but where does this equation come from, is it just from experiments?

The magnetic moment $m$ of a particle is given by $m=kS$, where $k$ is a constant known as the gyromagnetic ratio and $S$ is the particle's spin.

But where does this equation come from? Is it just from experiments?