The single most important thing that will matter the most will be your level of
honesty with yourself & with your therapist. Any therapist who thinks you may be hiding something either from yourself or from them (or both) is likely to be unclear about your motives & may hold back from making any diagnosis. This is not the 20th century (thankfully), we do not have to present a perfect lie in order to get treatment (in many parts of the world - some places do still seem to be stuck in earlier times).
Note: I live in Philly, where we have the Mazzoni Centre. PA allows
Informed Consent for HRT & Mazzoni uses that, so there is no requirement for any letter from a therapist, doc, whomever. If you happen to live in a part of the world that still demands that "perfect story" from 20th century thinking, then ignore everything I say & do what Sarah said.
I would suggest going to the therapist with a presentation you are comfortable with (eg: I am still too uncomfortable to present female even after 7 months on E).
I have presented male at every therapy appointment. At my first session (with my wife) I was even wearing a bushy goatee & sideburns. The therapist had to work a little to get me to shave them (my internal transphobia level is kinda high). Granted, the prompt to call for an appointment was an ER visit caused by self-medding. That may have made a difference with how the therapist dealt with me.
I was recommended to make an appointment for HRT after 6-7 weeks.
Much of how things go will depend on where you live. Check to see if your state/country allows Informed Consent. If it does, then no therapist is required. If not, then find out exactly what hoops you will need to go through to get to where you need to get to.
Best of luck

Tessa
PS: I am still seeing my gender therapist regularly (down to every 2 weeks since the start of this year), but by now we are spending most of the time dealing with issues other than my gender dysphoria. Hiding so thoroughly from myself for 38 years left me with a bit of emotional baggage that has been proving difficult to shed.