Warning msgbox when closing a userform

[JamesT1]

Hi
I have a userform for inputing a range of vehicle details. It will not let the user add the details unless all fields are complete.

If the user complets some of the fields and closes the form it will not save any information.

They must use a command button to close the form (the red X will not show)

I am looking for a way for a warning msg box to appear when they select the close form button

thanks