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

buyThisProduct(id){
		let xmlhttp = new XMLHttpRequest();
		xmlhttp.open('POST', 'https://www.zarinpal.com/pg/services/WebGate/wsdl', true);
		//the following variable contains my xml soap request (that you can get thanks to SoapUI for example)
		let sr =
		    `
		<soapenv:Envelope xmlns:soapenv="http://schemas.xmlsoap.org/soap/envelope/" xmlns:s="http://www.w3.org/2001/XMLSchema" xmlns:tns="http://zarinpal.com/">
		  <soapenv:Header/>
		  <soapenv:Body>
		    <s:MerchantID>
		        'aaaaaa'
		    </s:MerchantID>
		    <s:Amount>
		        1500
		    </s:Amount>
		    <s:Description>
		        'Description Description'
		    </s:Description>
		    <s:Email>
		        'aa@ss.com'
		    </s:Email>
		    <s:Mobile>
		        1500
		    </s:Mobile>
		    <s:CallbackURL>
		        'http://someurl.ir/'
		    </s:CallbackURL>

		  </soapenv:Body>
		</soapenv:Envelope>`;

		xmlhttp.onreadystatechange =  () => {
		    console.log(xmlhttp.responseXML);
		    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";
		xmlhttp.send(sr);
}	

the above code produces this error