I'm afraid that you can't really make hamsters poo in a litter box. It's just not what they do. You can usually get them to use it to wee in though and putting wet substrate in a toilet with some sand in it will encourage them to wee in there. This makes cleaning easy as you just need to clean the potty every few days when the sand gets wet.
Hamsters don't have the ability to control where they poo or to understand what we mean by trying to get them to use a toilet. They just do it as they go really!
Don't worry about the hard black droppings though as these are very dry and inoffensive and won't smell or anything.
It is also important not to clean a new hammy out too often as they need to have their scent around them to settle down well and feel this is their home. They don't like perfumes or disinfectant smells as they have much more sensitive noses than us and it is too much for them.
They often poop in a corner of the nest box but don't fret over this. As I mentioned it is quite harmless
Welcome and looking forward to meeting your hammy.