The ornamental design for a watersupplythroughway for a urinal, as shown and described.
FIG. 1 is a top, front, left perspective view of a water supply throughway for a urinal showing our new design;
FIG. 2 is a top view thereof;
FIG. 3 is a front view thereof;
FIG. 4 is a left side view thereof; and,
FIG. 5 is a right side view thereof.
The broken lines immediately adjacent to the shaded area represent unclaimed boundaries of the design. The broken lines shown in the remainder of the drawings depict environment only that forms no part claim.