The ornamental design for an writinginstrumentholder, as shown and described.
FIG. 1 is a front elevation view of the writing instrument holder showing my new design;
FIG. 2 is a rear elevation thereof;
FIG. 3 is a left side, elevation view thereof, the right side being a mirror image of the left side;
FIG. 4 is a top plan view thereof;
FIG. 5 is a bottom plan view thereof; and,
FIG. 6 is a right side, perspective view thereof.
The broken lines showing two writing instruments in FIG. 6 are included for the purpose of showing articles in the environment that form no part of the claimed design.