You could either add the default text to the Text property of the combobox like this (preferred):
ddlCategory.Text = "Add new Category";
Or, you could add the value to the datatable directly:
if you are using datatable
da.Fill(dt);
DataRow row = dt.NewRow();
row["Category"] = "Add new Category";
dt.Rows.InsertAt(row, 0);
ddlCategory.DataSource = dt;
}
if you are using list of class
objProdCat=getinglist
objProdCat.Insert(0,"Add new Category");
ddlCategory.DataSource=objProdCat.SelectAll();