I claim the ornamental design for a key for attachment to a ring, as shown and described.
FIG. 1 is a front perspective view of a key for attachment to a ring showing my new design, the broken-line showing of a ring is for illustrative purposes only and forms no part of the claimed design;
FIG. 2 is a front elevational view of the key.
FIG. 3 is a top view thereof.
FIG. 4 is a bottom view thereof.
FIG. 5 is a left side elevational view thereof; and,