The ornamental design for a necktie accessory, as shown and described.
FIG. 1 is a front-side perspective view of a necktie accessory;
FIG. 2 is a rear-side perspective view of the necktie accessory;
FIG. 3 is a front elevation view of the necktie accessory;
FIG. 4 is a rear elevation view of the necktie accessory; and,
FIG. 5 is a left side elevation view of the necktie accessory.
The broken line showing of a necktie accessory is included for the purpose of illustrating environmental matter and forms no part of the claimed design.