You need to book the Fantasmic! Dining Package specifically. Just a regular reservation at one of the 3 restaurants will not get you the F! voucher.
The package is for the first F! showing and in January that will probably be early since sunset is early. F!DP times will probably won't be any later than 5:00.
Sometimes F!DP times aren't in the online system right at 180 days. You can call and see if the CM can make it for you.
F!DP is a fixed price per person. At HBD you are entitled to an appetizer, entree, non-alc beverage and dessert ordered from the regular menu. At MM the same entitlements, but the dessert is a pre-plated sampler platter served family style. H&V is a buffet.