I would wait. Seastars and relatives are some of the most delicate when it comes to osmotic balance. These parameters are changing dramatically in a new tank but you do not test for it. Similarly, your skills as a hobbyist must develop in keeping parameters stable.
As an aside, I would never, ever recommend a sand sifting star as most will starve in all but the largest tanks (with open sand beds) in about a year. In addition, they eat beneficial creatures in a sand bed.