How to pass the value of a client variable to a Node.JS server?

Question

I am developing an application that will need to replace an XML via FS, and all the contents of that XML is stored in a variable that is created by JS in the client after pressing a button. I need to get the value of this variable from within the Node.JS server, to use as a parameter for the function writeFile() from FS. How do I do this?

Client JS:

 let xml=(`
 <?xml version="1.0" encoding="UTF-8"?>
 <Exemplo>
  <Titulo>Hello World</Titulo>
 </Exemplo>
`)

Server JS:

const server = http.createServer((request,response)=>{
 fs.writeFile('dataset.xml',QUERO_A_VARIAVEL_AQUI,function(err){
  if(err) return console.log(err);
 })
})

I have tried using import and export, but got errors that variable cannot be accessed after startup. I tried to play the function of generating XML inside the Server.JS file and calling it in HTML being activated by onclick="" button, but still no value is accessed by the server, and if I put the function inside the server it is as if it simply did not exist.