Datbound ListBox Select Multiple Items

 I have a ListBox tied to SqlDataSource.
Here is the data source: <asp:SqlDataSource ID="SqlDataSource3" runat="server" ConnectionString="<%$ SomeConnection %>" SelectCommand="SELECT [AmenityID], [AmenityName] FROM [Amenity_Table]"> </asp:SqlDataSource> Here is the ListBox: <asp:ListBox ID="amenity" runat="server" DataSourceID="SqlDataSource3" DataTextField="AmenityName" DataValueField="AmenityID" SelectionMode="Multiple" Width="70%" Rows="7">
 
Now what I have is a query that returns a string of existing items that need to be selected. So a seperate data source returns a string that I split and this contains the AmenityID. I have this in my page load ...