Hello,
you sound very confused.
If your car has an Arduino mounted and you control it remotely from your PC, why should the Arduino provide you with keyboard codes for a device it doesn't have? That sounds more like you should look up in the documentation of your PC-Programming platform and not on the Arduino side how to handle keyboard input.
As a general note, you should first clean make at least a rough concept about how your system is going to work. Get rid of all the "Magic happens here" parts. My guess is you're going to need about 4 ports in your software:
[1] On your PC something to handle the key commands you issue.
[2] On your PC something to send the commands read in Step 1 to the Arduino
[3] On the Arduino something to handle the received commands from the PC created in Step 2
[4] On the Arduino something to execute the received commands from step 3 on the car hardware to change direction or speed.
Each of these modules can be refined and tested individually, usually quite independent of the rest. In Step [1] you will work only on your PC and work on the user interface, Step [4] will deal with your car hardware and how to attach it to the Arduino, in Steps [2] and [3] you will mess around with the communication between the PC and the Arduino.
Good luck.
Korman