Add a new node in existing XML file

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C#

ASP.NET

XML

Hi all,
i want to add a new node in my existing "XML" file. my xml file structure is like:

<Response>
<Result>SUCCESS</Result>
<Response>
<EmpLoginAPI>
<EmployeeID>214735</EmployeeID>
<OrganizationID>1</OrganizationID>
<CompanyID>81</CompanyID>
<MobileNo>9899912648</MobileNo>
</EmpLoginAPI>
</Response>
</Response>

My expected output is:

<Response>
<Result>SUCCESS</Result>
<Response>
<EmpLoginAPI>
<EmployeeID>214735</EmployeeID>
<OrganizationID>1</OrganizationID>
<CompanyID>81</CompanyID>
<MobileNo>9894672648</MobileNo>
<pre><otp>test</otp>

// this is newly added element







but when i try the given code, i face the error below,

XmlDocument xmlDoc = new XmlDocument();
               xmlDoc.LoadXml(ResXml);
               XmlNode EmpLoginAPI = xmlDoc.SelectSingleNode("//EmpLoginAPI");
               XmlNode MobileNo = xmlDoc.SelectSingleNode("//MobileNo");
               XmlNode xmlRecordNo = xmlDoc.CreateNode(XmlNodeType.Element, "otp", null);
               xmlRecordNo.InnerText = "test";
               EmpLoginAPI.InsertAfter(xmlRecordNo, MobileNo);
               xmlDoc.Save(ResXml);

// here am getting error

error:

Illegal characters in path.

please drop me a solution for this
thanks in advance.

What I have tried:

add a new node in existing xml file