Adding default value to select when the items are dynamic

I am populating my selectlists in the lightbox from a database table using C# and Linq, how do I add
default values to my selects … i.e. “Please select” and make an item mandatory?

var costCodeList = costCodes.Select(costCode => new { key = costCode.UNIQUE_ID, label = costCode.DESCRIPTION }).ToList();

Sorry that was meant to be a code block not URL ^