App request post data encoding charset

I’m not able to change the charset encoding to anything diffrant than UTF-8, my application supposed to send the data to the server with charset=windows-1256, as all backend services is using this charset

I trying by setting the contentType to application/x-www-form-urlencoded;charset=windows-1256 but F7 is always seding with charset=UTF-8

I also tried with the method beforeSend, but also I got the same result

how I can change the POST data charset?

example as below:

app.request({
  url: 'aaaa',
  method: 'POST',
  contentType: 'application/x-www-form-urlencoded;charset=windows-1256',
  data: {
	reqCode:'send','clientType':'JSON'
  },
  beforeSend: function (xhr) {
	console.log(xhr);
	console.log('Before');
	xhr.setRequestHeader( 'Content-type', 'application/x-www-form-urlencoded;charset=windows-1256');
	console.log('After');
	console.log(xhr);
  },
  success: function (data) {
	let response = JSON.parse(data);
		app.dialog.alert(response);
  },
  error: function (data) {
	console.log("ERROR");
	console.log(data);	
	   app.dialog.alert("Error",response);					        
  }
});

for more clarification

request headers are sent as below
image

see Content-Type how it was modified and has 2 recordss, and also with wrong charset, not the one I set in the request headers:
xhr.setRequestHeader( ‘Content-type’, ‘application/x-www-form-urlencoded;charset=windows-1256’);

Thanks for the help