Open Device Manager in your laptop.
Expand the Keyboards section.
You might be seeing this (if you have not yet connected the external keyboard) :-
The keyboard arrow marked in red is the inbuilt keyboard your laptop has. (Your laptop's keyboard might have a different name)
Now connect your wireless keyboard.
You will see another keyboard added under the keyboards section in device manager (arrow marked in green).
Again, your wireless keyboard would have a different name than mine (mine is wired). This is just an example.
Now right click on the inbuilt keyboard and click 'uninstall' (marked with red arrow).This will disable your inbuilt keyboard.
Do not panic as the inbuilt keyboard's drivers would be reinstalled as soon as you restart the machine. However, uninstall the inbuilt one only after you have plugged in your wireless one.