Add values to a list contains a Lookup Field

protected void AddEmp(object sender, EventArgs e)
{
            string emp_name = txtEmployeeName.Text;
            string emp_designation = ddlDesignation.SelectedValue;
                    
    using (SPSite site = new SPSite("My Site"))
            {
                using (SPWeb web = site.OpenWeb())
                {
                    SPSecurity.RunWithElevatedPrivileges(delegate()
                    {
                        web.AllowUnsafeUpdates = true;
SPList splist_employees = web.Lists["Employee"];

SPList splist_designations = web.Lists["Designations"];

                        SPListItemCollection splc_items = splist_employees.Items;
                        SPListItem spli_item = splc_items.Add();
                        SPFieldLookup lookup = splist_employees.Fields["Designated"] as SPFieldLookup;

                        string fieldName = lookup.LookupField;
                        spli_item["Employee Name"] = emp_name;
                        spli_item[fieldName] = GetLookFieldIDS(emp_designation, splist_designations);

                        spli_item.Update();

                    });
                }
            }

}

        public static string GetLookFieldIDS(string lookupValues, SPList lookupSourceList)
        {
            string id = string.Empty;
            SPFieldLookupValueCollection lookupIds = new SPFieldLookupValueCollection();
            SPQuery query = new Microsoft.SharePoint.SPQuery();
            query.Query = "<where><eq><fieldref name="Designation" /><value type="Text">" + lookupValues + "</value></eq></where>";
            SPListItemCollection listItems = lookupSourceList.GetItems(query);
            foreach (Microsoft.SharePoint.SPListItem item in listItems)
            {
                id = item.ID.ToString();
            }
            return id;
        }