To whom it may concern:
(your name) is a patient who has been under my care since (when you began seeing the doctor). I have first-hand knowledge of (your first name)‘s disability and the functional limitations caused by the disability. Due to (his/her) disability, I am prescribing an emotional support dog to assist with limitations caused by (symptoms of disability experienced, such as stress or anxiety).
I will be happy to answer any questions you may have regarding my recommendation that (your name) have an emotional support dog.