Retaining same amount of digits

WM_CHAR takes a virtual key, NOT text...

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Originally Posted by MSDN

This message is posted to the window with the keyboard focus when a WM_KEYDOWN message is translated by the TranslateMessage function. WM_CHAR contains the character code of the key that was pressed.

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You will need to use a custom message for custom input. WM_USER is made for that task. Don't use any of the messages with a specific task because the default window procedure might cause your program to behave strange.
Also I'm not sure how WPARAM and LPARAM are defined but whenever a pointer is to be passed in a message in Windows API it usually goes in LPARAM (such as with WM_SETTEXT) so WPARAM might not be designed to be able to handle pointers.