Just call the :define-key method from a keymap object, and pass the command's name with its arguments in a tuple:

(def keymap (:new-keymap (in jwno/context :key-manager)))  # Create a new keymap
(:define-key keymap "Win + ," [:split-frame :horizontal 2]
             "Split current frame horizontally")

Here :split-frame is the name of our command. :horizontal and 2 are its arguments. If a command takes no argument, you can pass only the name, without putting it in a tuple:

(:define-key keymap "Win + Shift + Q" :quit
             "Tell Jwno to exit")

The code above is equivalent to:

(:define-key keymap "Win + Shift + Q" [:quit]
             "Tell Jwno to exit")