chrome.browserAction.onClicked.addListener(function(tab) {

chrome.tabs.getSelected(null,function(tab) {

    var link = tab.url;
    var ebaycode = link.replace("http://www.ebay.de/itm/","");
    var maxBidField = document.getElementById('MaxBidId');

    var x = new XMLHttpRequest();
    x.open('GET', 'http://*.*.*.*/GetMaxBid/' + ebaycode);
    x.onload = function() {
        var resp = x.responseText;
        resp = resp.replace('<string xmlns="http://schemas.microsoft.com/2003/10/Serialization/">',"");
        resp = resp.replace('</string>',"");
        alert(resp);

        //THE CODE THAT WILL FILL INTO FIELD
    };
    x.send();

});