I would not feed it chick starter, or layer mash. It’s not going to have enough niacin in it. I recommend ordering a duckling feed like the one Phantom mentioned which would be perfect for it. Or if you can’t find duck specific feed, purchase some flock raiser crumbles. They have that stuff everywhere!
Yes, if you’re going to keep this duckling, you need to take care of it properly. That means purchasing the right feed.