The RegisterWindowMessage method, given a string, returns a system wide unique message ID, suitable for sending messages between applications who both register the same string.
CommentsOnly use RegisterWindowMessage when more than one application must process the
same message. For sending private messages within a window class, an application
can use any integer in the range WM_USER through 0x7FFF. (Messages in this range
are private to a window class, not to an application. For example, predefined
control classes such as BUTTON, EDIT, LISTBOX, and COMBOBOX may use values in
Win32 API References