The ornamental design for a tensionkey, as shown and described.
FIG. 1 is a perspective view of a tension key;
FIG. 2 is a front view thereof;
FIG. 3 is a rear view thereof;
FIG. 4 is a right side view thereof;
FIG. 5 is a left side view thereof;
FIG. 6 is a top view thereof;
FIG. 7 is a bottom view thereof;
FIG. 8 is a rear perspective view thereof; and,
FIG. 9 is a perspective view of the design of FIG. 1 shown in a position of use mounted on a bracket of a curtain.
The broken lines showing a headrail, bracket, and attaching screw in FIG. 9 depict environmental subject matter that forms no part of the claimed design.