Call plugin using Ribbon Button in Dynamic CRM

Hi all,
I want to call plugin using custom ribbon button in Dynamic CRM online(Dynamic 365).
I have go though several link but still i am getting error.
My Code is

function CallActionFromJavaScript() {
debugger;
var entityId = Xrm.Page.data.entity.getId();
var entityName = "new_customsendactivity";
var requestName = "new_ActionCall";

var xml = "" +
"<s:Envelope xmlns:s=\"schemas.xmlsoap.org/.../envelope\"><s:Body><Execute xmlns=\"schemas.microsoft.com/.../Services\" xmlns:i=\"www.w3.org/.../XMLSchema-instance\"><request xmlns:a=\"schemas.microsoft.com/.../Contracts\"><a:Parameters xmlns:b=\"schemas.datacontract.org/.../System.Collections.Generic\"><a:KeyValuePairOfstringanyType><b:key>Target</b:key><b:value i:type=\"a:EntityReference\"><a:Id>"+entityId+"</a:Id><a:LogicalName>"+entityName+"</a:LogicalName><a:Name i:nil=\"true\"/></b:value></a:KeyValuePairOfstringanyType></a:Parameters><a:RequestId i:nil=\"true\"/><a:RequestName>"+requestName+"</a:RequestName></request></Execute></s:Body></s:Envelope>" +
"";

var xmlHttpRequest = new XMLHttpRequest();

xmlHttpRequest.open("POST", "cenacis.crm8.dynamics.com/.../web", true);
xmlHttpRequest.setRequestHeader("SOAPAction","schemas.microsoft.com/.../Execute");
xmlHttpRequest.setRequestHeader("Content-Type", "text/xml; charset=utf-8");
xmlHttpRequest.setRequestHeader("Accept", "application/xml, text/xml, */*");
//xmlHttpRequest.setRequestHeader("Content-Length", xml.length);
xmlHttpRequest.send(xml);

var resultXml = xmlHttpRequest.responseText;
alert(resultXml);
}

Below is my Plugin steps

And below is the plugin Code

I am getting the below error when call from button

Please suggest, how to resolve this issue.