XML namespaces are a pain. The
System.Xml.Linq
namespace makes life slightly easier:
XDocument xml = XDocument.Parse(ResponseAPI);
XNamespace nsAccVal = "http://www.iibcdm.T24systems.com/FTAccountValidation";
IEnumerable<XElement> xNodeList = xml.Descendants(nsAccVal + "IBANNumber");
string ibanNumber = (string)xNodeList.FirstOrDefault();
System.Xml.Linq Namespace | Microsoft Docs[
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