With sand sifting stars, most people recommend waiting until your tank is established before adding them - however, you might be able to target feed it with things like clam on half shell, oyster, snail or conch meat (a lot of places offer these frozen), a good quality omnivore food (like LRS Reef Frenzy) etc. It should swallow any of these foods whole, as long as the food isn’t too big for the star. They generally feed on detritus in our tanks, but target feeding might provide enough of the proper nutrition for them, and these are supposed to be some of the kinds of food they eat in the wild anyway. Plus, they are known for eating all of the detritus in our tanks and starving to death, so target feeding them larger food pieces relatively regularly might actually help it do better long term. No guarantees though.
If you decide to get it and try target feeding it, let us know how it turns out.