Either the general reservations email address or calling and asking to speak with the in-house reservations manager. I don’t accept the first rate they offer (which is sometimes no better than the rate online) and often go back and forth several times, particularly where another property is offering me a lower rate but I really want to stay at the property I’m talking to. They’ll often offer F&B, laundry, and other discounts too, and you’re free to negotiate almost anything! One Hyatt Regency gave me access to a free washer and dryer in the back of the house and brought a chaise lounger into the room. I’ve also had no problem borrowing higher quality office chairs from hotels and having them whitelist my devices so I never get internet login pages, making working from them easier.