incorporating paramaeter into "Your own SQL"

[Sayjak]

Hello,
I am a brand new user of Cognos and having difficulty with prompts.
I created a report using "my own SQL".. Works super.
I attempt to add a prompt and that's where things get difficult. I want to provide the user with the ( single column ) results of another query and then use that result in my main query.
something like:
main query:
  select blabla
  from tablea
  where substr( item_code, 1, 2 ) = ?Month?
Parameter query:
  Select month_code "Month"
  from month_table
Or whatever..
I keep getting pointed back at the "package" tree when dealing with prompts where I need to be pointed to my queries..
If that makes sense, I'd appreciate any help.
Thanks!