WGTF_TOOLTIP flag in the ulClassFlags field of
XCENTERWIDGETCLASS.
Only if that bit is set, your widget will receive a WM_CONTROL message
with the TTN_NEEDTEXT notification and XCENTER_TOOLTIP as
the control's ID.
You will need to include include\helpers\comctl.h from the
&xwp; helpers for the definitions of the &xwp; tooltip control.
Again, if you have the full &xwp; source code, see the widgets in
src\widgets for how this is done.