Damiano,
Try this:
#define BM_SETSTYLE 244
oDlg:bInit := { || oBtnBmp:SendMsg( BM_SETSTYLE, BS_DEFPUSHBUTTON, 1 ) }
or
oDlg:bInit := { || oBtnBmp:PostMsg( BM_SETSTYLE, BS_DEFPUSHBUTTON, 1 ) }
CONTROL "", 100, "Button", BS_BITMAP|BS_DEFPUSHBUTTON|WS_TABSTOP, 17, 20, 51, 41
CONTROL "", 101, "Button", BS_ICON|BS_DEFPUSHBUTTON|WS_TABSTOP, 77, 20, 51, 41