Well, from my experience is that there isn't one specific professional type which you have to see to have a referral. Based on local standards where I'm from, for the most part any medical professional who has received training on WPATH can complete the assessment and make the referral to an endocrinologist. This includes GPs, psychiatrists, psychologists, nurse practitioners, nurses, etc. Depending on what kind of therapist you have, even that person could do the referral provided they get up to speed on WPATH. A lot of times these "general" therapists will get you to seek out a gender therapist who already has the knowledge and experience instead of getting it themselves.
I hope this helps.