I have no idea about the touchpad, but if I press a key on the keyboard, it just shows up in the chatbox along with the cursor. It's really quick and easy.
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World of Warcraft added this feature and it was never used by the vast majority of players.
It still goes un-used for the most-part simply because there are better and more convenient alternatives.
I'm not sure how the PS4's touchpad figures into it since PS3 doesn't have one, but at least on PS3 the default is an option called Direct Chat. With that option set, you use the controller for everything except chat. Whenever you start typing on the keyboard it automatically shifts focus to the chat window and interprets your keystrokes as chat text.
(You can also turn the Direct Chat option off in Character Settings if you want to. That way you can use keyboard shortcuts, assigning actions to keystrokes the way most PC players do. Then when you want to chat you just need to manually select the chat window before doing so.)