The FMEVENT_TOOLBARLOAD message is sent to an extension DLL when File Manager is loading its toolbar. This message allows an extension DLL to add a button to the File Manager toolbar.
FMEVENT_TOOLBARLOAD lpfmstbl = (LPFMS_TOOLBARLOAD) lParam; /* toolbar data */
An extension DLL must return TRUE to add the button to the toolbar. If the DLL returns FALSE, File Manager does not add the button.