billing 90806 for LCSW for Medicare

Can anyone help me find where on Medicare's website I can find information on billing for psychotherapy for this type of provider? What diagnosis codes supports medical necessity? Does the refering provider need to be a physician? Can the LCSW be utilized as the refering provider?

billing for LCSW, diagnosis to support medical necesity

I found diagnosis info on the CMS website, but I still have the burning question of the refering provider field on the claim. Does this have to be a physician, or can this be polpulated with the LCSW themself?