c# - Inserting data at specific position in XML

Question

I want to read an XML file and match tag </contrib-group> and write a string after this tag

string Final = File.ReadAllText(Npath);
string Oxml = path + "\\" + Oword + ".abs.xml";
if (File.Exists(Oxml))
{
    StreamReader xml = new StreamReader(Oxml,Encoding.UTF8);
    string xmltag = xml.ReadToEnd();
    //File.OpenWrite(Oxml);
    xml.Close();
    StreamWriter write = new StreamWriter(Oxml, true, Encoding.UTF8);
    Match tag = Regex.Match(xmltag, @"</contrib-group>");
    if (tag.Success == true)
    {
        write.WriteLine(Environment.NewLine);
        write.Write(Final);

    }
}

So I need to write the string Final to the XML file called Oxml after the matched XML tag </contrib-group>