At the top of their profile page, is a couple of tabs, "ADD A FRIEND" is one of them
Click that, and wait for them to accept the request. If so, return to player profile, and same place, "SEND MESSAGE" will have appeared.
Click it, a "PM" Private Message, box shall appear. Write your message and send it to them. If they respond, on your profile a green box will appear, notifying you, that you have a message