In order to do this go to Studio - Studio setup - Mackie Control (the one for your device).
The box beneath is where you can program your own controls. These are generic columns and do not change per device. Therefore, if the device had 4 function buttons, Cubase would still display 8. User A and User B are not mapped.
Let’s try to program some of our own functions;
To program ‘Add Track’, in the category drop down list and then find ‘Audio’ or ‘Audio Mono’ on the command drop down list on say the ‘F2’ row as is depicted in the screen shot. Click ‘Apply’. F2 is now programmed as your ‘Add audio track’ button.
You can program a second command for the same button as depicted in the next picture i.e. ‘Add Instrument Track. You would need to hit SHIFT and F2 to enable this. Of course you can choose another button altogether as you see fit for 'Add Instrument Track'.
Make sure you click Apply regularly as you update your function key list.
There are many programming options here for your function buttons - a total of 16 separate function - explore and enjoy!