How to add node prefix to an existing request XML string in C#?

You can use an XML library like 'XDocument' Class, using 'System.Xml.Linq' - MS Learn | XDocument Class[^]

You need to load your existing XML string into an 'XDocument'.
You can then modify the element names by adding the prefix 'pmd'.
Then convert your modified 'XDocument' back to a string -

using System;
using System.Xml.Linq; //See my link for a proper tutorial and sample code...

class Program
{
    static void Main()
    {
        //Your existing XML string...
        string existingXml = @"<CREATEPMDEBTORMANDATE_FSFS_REQ>
                                <FCUBS_HEADER>
                                    <SOURCE>DDS</SOURCE>
                                </FCUBS_HEADER>
                                <FCUBS_BODY>
                                    <Dr-Dd-Mandate-Master-Full>
                                        <CR_AC_CCY>AED</CR_AC_CCY>
                                        <CR_NAME>FOOD SUPPLIES LLC</CR_NAME>
                                    </Dr-Dd-Mandate-Master-Full>
                                </FCUBS_BODY>
                            </CREATEPMDEBTORMANDATE_FSFS_REQ>";

        //Load the existing XML string into an XDocument...
        XDocument xdoc = XDocument.Parse(existingXml);

        //Add the 'pmd:' prefix to the element names...
        AddPrefix(xdoc.Root, "pmd");

        //Convert your modified XDocument back to a string...
        string modifiedXml = xdoc.ToString();

        //Print the modified XML or use where you need it...
        Console.WriteLine(modifiedXml);
    }

    //Recursive method to add the prefix to all elements and their descendants...
    static void AddPrefix(XElement element, string prefix)
    {
        element.Name = XName.Get(prefix + ":" + element.Name.LocalName, element.Name.NamespaceName);
        foreach (var child in element.Elements())
        {
            AddPrefix(child, prefix);
        }
    }
}