midi keyboard

I would start by reading the Handbook under "Note input". Using the MIDI keyboard is no different form clicking the notes on the staff or typing them, you use the same process of first selecting the duration then entering a pitch.

First: You have to connect the external MIDI keyboard to your computer.

Second: You have to open Musescore app.

Third: You have to set as "ON" the MIDI input symbol of the Musescore toolbar menu (The standard 5 pins DIN male connector).

BYW: Musescore doesn't have a full real time external MIDI input. I mean; you cannot to play a piece expecting to get the full final score, not. You have to input note by note, one step at a time.