How to write xml declaration using Mule DataWeave?

I need to do add standalone=yes to XML declaration using DataWeave. I have tried different approaches, but I can't get it resolved.

current output which I'm getting:

<?xml version='1.0' encoding='UTF-8'?>
<Input:xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="csm_admin.xsd">
  <add>
    <data>
      <@firstName>jane</@firstName>
      <lastName>ALDANA</lastName>
    </data>
  </add>
  <add>
    <data>
      <@firstName>Emma</@firstName>
      <lastName>EDWARDS</lastName>
    </data>
  </add>
</Input>

This is my Expected xml output:

<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<admin xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:noNamespaceSchemaLocation="csm_admin.xsd">
  <add>
    <data>
      <@firstName>jane</@firstName>
      <lastName>ALDANA</lastName>
    </data>
  </add>
  <add>
    <data>
      <@firstName>Emma</@firstName>
      <lastName>EDWARDS</lastName>
    </data>
  </add>
</Input>

I tried to add standalone=yes declaration by concatenating but it is failing too.
Thanks for your input

Here's one approach you can try:

1. Convert the XML payload to a string without including the existing declaration. Then, concatenate the formatted string with the new declaration. In DataWeave, set the output type to text/plain.
2. Use "Set Payload" to change the MIME type to application/xml in order to get the desired XML output.

Transform Message

%dw 2.0
var in = write(payload,"application/xml", writeDeclaration:false)
output text/plain
---
'<?xml version = "1.0" encoding="UTF-8" standalone="yes"?>' ++ in

Set Payload

<set-payload value="#[payload]" doc:name="Set Payload" doc:id="664c8d66-0bd7-4185-b754-1c4bc932221e" mimeType="application/xml"/>

Input

<?xml version='1.0' encoding='UTF-8'?>
<employees>
  <employee>
    <name>
      <lastname>Kelly</lastname>
      <firstname>Grace</firstname>
    </name>
    <hiredate>October 15, 2005</hiredate>
    <projects>
      <project>
        <product>Printer</product>
      </project>
    </projects>
  </employee>
</employees>

Output

<?xml version = "1.0" encoding="UTF-8" standalone="yes"?><employees>
  <employee>
    <name>
      <lastname>Kelly</lastname>
      <firstname>Grace</firstname>
    </name>
    <hiredate>October 15, 2005</hiredate>
    <projects>
      <project>
        <product>Printer</product>
      </project>
    </projects>
  </employee>
</employees>