How do I pass a hidden field value into my sql command?

I have a datareader that I want to pass a hidden variable into the sql statement, but I'm not sure how to use the hiddenfield value.  Here's a portion of my code behind...  what am I forgetting? 
comm = New SqlCommand("SELECT lastname, firstname from tMatters where  MatterID = @hdnMatterID;", conn)
comm.Parameters.AddWithValue("hdnMatterID)
...