Populate dropdownlist with secondary

I have a dropdownlist that chooses a description of a major category. The code works by sequence, If I choose a major sequence  i want to populate the secondary dropdownlist with all the minor categorys. Not to sure how to make this happen.

If ddCatsMinor.SelectedIndex + 1 = ddCatsMinor.Items.Count Then
                EndingValue = 99999999
            Else
                EndingValue = ddCatsMinor.Items(ddCatsMinor.SelectedIndex + 1).Value
            End If
            Response.Redirect("SRP.aspx?i=" & Format(Val(ddCatsMinor.SelectedValue), "00000000") & "2" & Format(EndingValue, "00000000"))
        End If
 
 
''########################################################
Private Sub SetupOBItems()
 
 
        Dim cmd As SqlCommand
        Dim prm As SqlParameter
 
        cmd = New SqlCommand("spA1SGetItemsForOBHeader", conn)
        cmd.CommandType = CommandType.StoredProcedure
 
        prm = New SqlParameter("@BegSeq", SqlDbType.Int)
        prm.Value = Mid(Request.QueryString("i"), 1, 8)
        cmd.Parameters.Add(prm)
 
        prm = New SqlParameter("@EndSeq", SqlDbType.Int)
        prm.Value = Val(Mid(Request.QueryString("i"), 10, 8)) - 1
        cmd.Parameters.Add(prm)
 
        Dim oRDR As SqlDataReader
        cmd.CommandTimeout = 120 'Number of seconds to wait for a response.
 
        conn.Open()
        oRDR = cmd.ExecuteReader(CommandBehavior.Default)
 
        While oRDR.Read
            cIdx += 1
            ReDim Preserve Items(cIdx)
            Items(cIdx) = oRDR("Product")
        End While
 
        'Clean Up
        If (Not oRDR Is Nothing) Then
            If (oRDR.IsClosed) Then
                oRDR.Close()
            End If
        End If
        If (Not conn Is Nothing) Then
            If conn.State <> ConnectionState.Closed Then conn.Close()
            conn.Dispose()
        End If
        If Not cmd Is Nothing Then cmd.Dispose()
 
    End Sub
 
 
''"###############################################
Private Sub LoadOBCats()
        Dim ar As ListItem()
        Dim ku As Long = 0
 
 
        Dim cmd As SqlCommand
        Dim prm As SqlParameter
 
        cmd = New SqlCommand("spA1SGetOBHeaders", conn)
        cmd.CommandType = CommandType.StoredProcedure
 
        Dim oRDR As SqlDataReader
        cmd.CommandTimeout = 120 'Number of seconds to wait for a response.
 
        conn.Open()
        oRDR = cmd.ExecuteReader(CommandBehavior.Default)
 
        While oRDR.Read
            ddCats.Items.Add(oRDR("Description"))
            ddCats.Items(ddCats.Items.Count - 1).Value = oRDR("OrderBookSequence")
 
        End While
        For Each li As ListItem In ddCats.Items
            ReDim Preserve ar(ku)
            ar(ku) = li
            ku += 1
        Next
        Dim ar1 As Array = ar
 
        ar1.Sort(ar1, New ListItemComparer)
        ddCats.Items.Clear()
        'Add an item that will appear as the default selected item in the dropdown list
        ddCats.Items.Add(New ListItem("Select one", "Select one"))
        ddCats.Items.AddRange(ar1)
        'Clean Up
        If (Not oRDR Is Nothing) Then
            If Not (oRDR.IsClosed) Then
                oRDR.Close()
            End If
        End If
        oRDR = Nothing
        If (Not conn Is Nothing) Then
            If conn.State <> ConnectionState.Closed Then conn.Close()
            conn.Dispose()
        End If
        If Not cmd Is Nothing Then cmd.Dispose()
    End Sub

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