SSRS - Passing parameter filters from external source

When executing SSRS reports via report manager from an external website (Java based) which is handling the user security and report selection, how do you filter the available values in a report parameter (drop-down, etc.) appearing in the report via the values passed to the report? If you have multiple parameters, what is the best method?

I'm not sure how the external website is related.  Maybe they are and I'm not understanding how it's relative so please correct me if i'm wrong but couldn't you have another dataset in the report that populates the dropdown box for you?