Soap service call

I’m trying to use the PaymentRequest from this which is a banking gateway and i realy do need some help
it wont work with normal ionic http post which should be duo to the soap thing. so i searched a lot and could not find any official ionic way of doing this… i just found something that seemed to work for the case… but could not get that working either. here it is

		let xmlhttp = new XMLHttpRequest();'POST', '', true);
		//the following variable contains my xml soap request (that you can get thanks to SoapUI for example)
		let sr =
		<soapenv:Envelope xmlns:soapenv="" xmlns:s="" xmlns:tns="">
		        'Description Description'


		xmlhttp.onreadystatechange =  () => {
		    if (xmlhttp.readyState == 4) {
		        if (xmlhttp.status == 200) {
		            let xml = xmlhttp.responseXML;
		            let response_number = parseInt(xml.getElementsByTagName("return")[0].childNodes[0].nodeValue); //Here I'm getting the value contained by the <return> node
		            console.log(response_number); //I'm printing my result square number
		// Send the POST request
		xmlhttp.setRequestHeader('Content-Type', 'text/xml');
		xmlhttp.responseType = "document";

the above code produces this error