We claim the ornamental design for an accessory element for window system, as shown and described.
FIG. 1 is a perspective view taken from the front, bottom and right side of an accessory element for window system, showing the design, on the underside of a window system;
FIG. 2 is a perspective view taken from the front, top and right side of the accessory element for window system of FIG. 1; and,
FIG. 3 is a bottom plan view of the accessory element for window system of FIG. 1.
The left side of the accessory element for window system of FIG. 1 is a mirror image of the right side, and the rear of the accessory element for window system of FIG. 1 is a mirror image of the front.
The broken lines of the drawings disclose environmental structure that forms no part of the claimed design.