With Parsec, you can play A Hat In Time: Seal The Deal co-op with a friend or a stranger in Party Finder. Please note, however, that just like if you were playing with a friend sitting at your computer with you, you must "plug-in" your friend's controller before starting the game.
To virtually plug-in a controller, make sure you've installed the host software for controllers on Parsec (it's part of the installation process). Then when your friend connects, make sure you give him controller access. After that, your friend should hit any button on his controller. After that, start A Hat In Time.
You can enter the co-op mode of A Hat In Time after the opening credits. Just press START and choose Co-op.