How do I Concatenate an XML String in Structured Text

<s:Envelope xmlns:s="http://schemas.xmlsoap.org/soap/envelope/">
  <s:Header>
    <Action s:mustUnderstand="1" xmlns="http://schemas.microsoft.com/ws/2005/05/addressing/none">http://tempuri.org/IService/receiveHeartbeat</Action>
  </s:Header>
  <s:Body>
    <receiveHeartbeat xmlns="http://tempuri.org/">
      <reader>

*Word Variable*

</reader>
    </receiveHeartbeat>
  </s:Body>
</s:Envelope>

Reader_ID: STRING := '0';
	msg: STRING(500);
	Msg1: STRING(250) := '<s:Envelope xmlns:S="http://schemas.xmlsoap.org/soap/envelope/"><s:Header>';
	Msg2: STRING(250) := '<Action s:mustUnderstand="1" xmlns="http://schemas.microsoft.com/ws/2005/05/addressing/none">http://tempuri.org/IService/receiveHeartbeat</Action>';
	Msg3: STRING := '</s:Header><s:Body><receiveHeartbeat xmlns="http://tempuri.org/"><reader>';
	MsgAfter: STRING := '</reader></receiveHeartbeat></s:Body></s:Envelope>';

msg := CONCAT(Msg1,Msg2);
msg := CONCAT(msg,Msg3);
msg := CONCAT(msg,Reader_ID);
msg := CONCAT(msg,MsgAfter);