The ornamental design for a curtainholdbacktie, as shown and described.
FIG. 1 is a perspective view of a curtain holdback tie showing 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;
FIG. 7 is a bottom view thereof;
FIG. 8 is a perspective view of another state of the curtain holdback tie thereof; and,
FIG. 9 is an enlarged view of the selected portion in FIG. 1.
The broken lines shown in the drawings depict portions of the curtain holdback tie in which the design is embodied that form no part of the claimed design.