It's been years since I made Pt(NH3)4Cl, and unfortunately, my lab books are all packed up as we are moving to another building. I'm pretty sure the synthesis is listed in one of the early volume of "Inorganic Synthesis" by printed by Wiley & Co., but I no longer have access to that book. It's not an uncommon book in chemistry libraries. I'm think you are going to have to reduce the Pt (in the H2PtCL6) to the +2 state as the first step, since I believe the prep starts out with H2PtCl4. IIRC, the prep does involve the addition of aqueous ammonia, but I do not remember the steps at all. You will definitely want to find a written prep somewhere as just adding ammonia isn't going to give you what you want.