The HELPMSGSTRING message is sent by a common dialog box to its owner's window procedure whenever the user chooses the Help button. This message lets an application provide custom Help for the common dialog boxes.
wParam
Not used.
lParam
Points to the structure that describes the common dialog box.
The application returns zero.
To use the HELPMSGSTRING message, the application must create a message identifier by using the RegisterWindowMessage function and passing the HELPMSGSTRING constant as the function's single parameter.
In addition to creating a new message identifier, the application must set the hwndOwner member in the appropriate data structure for the common dialog box. This member must contain the handle of the window to receive the HELPMSGSTRING message.
The application can also process the request for Help in a hook function. The hook function would identify this request by checking whether the wParam parameter of the WM_COMMAND message was equal to psh 15.