<AssetWarrantyResponse>
 <AssetWarrantyResponse>
  <AssetEntitlement>
   <EndDate>2007-03-23T23:59:59</EndDate>
   <EntitlementType>INITIAL</EntitlementType>
   <ItemNumber>000-0000</ItemNumber>
   <ServiceLevelCode>TS</ServiceLevelCode>
   <ServiceLevelDescription>ProSupport</ServiceLevelDescription>
   <ServiceLevelGroup>20</ServiceLevelGroup>
   <ServiceProvider>DELL</ServiceProvider>
   <StartDate>2004-03-23T00:00:00</StartDate>
  </AssetEntitlement>
 <AssetWarrantyResponse>
<AssetWarrantyResponse>

foreach (string serviceTag in cleanServiceTags)
{
    var xmlDocument = XDocument.Load(dellApiUrl.Replace("<SERVICE_TAG>", serviceTag));
    var values = xmlDocument
                .Descendants("AssetEntitlement")
                .Select(x => new { endDate = (string)x.Element("EndDate"), entitlement = (string)x.Element("EntitlementType") });

    textBox1.Text = values.ToString();
}

using (WebClient assetLookup = new WebClient())
{
    xmlReply = assetLookup.DownloadString(dellApiUrl.Replace("<SERVICE_TAG>", serviceTag));
}

https://api.dell.com/support/assetinfo/v4/getassetwarranty/<service_tag>?apikey=<apikey>