A method for mounting an auxiliary member on a window or door frame already installed and whereon is mounted a drive assembly comprising at least one actuating member and at least one transmission rod mounted in at least one slot of the frame. The method comprises the steps of, providing, between the auxiliary member and the rod, a coupling that is slidable in a longitudinal direction and fixed in any direction orthogonal to the direction of sliding, and forming a through hole in the rod by a screw carried by the auxiliary member, and fastening the auxiliary member to the rod by the engagement of a tip of the screw with the through hole.