The keystream is as long as the message, so you don't want to send that to the receiver. The keystream is generated by a much shorter key (e.g., 128-bit private key). That is what the receiver needs in order to decrypt the message.
So, how do you get that to the receiver? Well, clearly you can't just send it in the clear or anyone watching the channel would have the key and could decrypt. Typically what we do in cryptography is run a key establishment protocol. This could be as simple as Alice and Bob meet up and exchange the key on a written piece of paper or as complex as using asymmetric crypto, with public/private key pairs, certification authorities, etc.