The ornamental design for hair extension holder, as shown and described.
FIG. 1 is a perspective view of the hair extension holder according to my new design;
FIG. 2 is a front view thereof;
FIG. 3 is a back view thereof;
FIG. 4 is a left side view thereof;
FIG. 5 is a right side view thereof;
FIG. 6 is a top view thereof; and,
FIG. 7 is a bottom view thereof.
The dot-dash lines on the surface of the hair extension holder represent stitching.
The stitching shown in dot-dash lines is part of the claimed design.
The dashed lines, which are of equal length in the drawings, illustrate the environment of the hair extension holder and form no part of the claimed design.