Could be found very easily. Go to Tripadvisor. Lookup the hotel. On top of the description page for the hotel, there's an 'E-mail hotel' link. This doesn't work with every hotel, but it does with this one. (It's something like "Amsterdam Zuidoost at Hilton dot com").

If a general manager is the least person you need to communicate with, look at the Tripadvisor reviews. His name will be in the comments. Then google "name" + @hilton.com. You'll find his Hilton-address on a travel employment website.