Similar to what was mentioned in other questions, these businesses offer them in various ways. In the case of this casino, you can get it through various means similar to any other place you might be going to.
You could get it as part of signing up with them.
You could get them through codes that other sites have provided.
Depending on how their loyalty program is structured, you might be able to get them.
Or you are attending a specific event where these are offered out for whatever kind of reason.