first of all you can not call your webservice method by simply write it's name and passwing parameter list like this....
GetFields(fieldName, _ID, myEditor.onGetFieldsCompleted, myEditor.onGetFieldsFailed);
you can call your serivce by using one of the most common method of jquery $.ajax() like this.
function callWebService() {
var _data = "{mergeFieldName:'YOUR VALUE',PartnershipID:'YOUR VALUE'}";
var _url = "YOUR SERVICE URL"
$.ajax({
type: "POST",
url: _url,
data: JSON.stringify(_data),
contentType: "application/json; charset=utf-8",
dataType: "json",
success: function (msg) {
alert(msg);
},
error: function (msg) {
alert('error' + msg);
}
});
for more detail on how to call webservice method using javascript or jquery please refer this links
Jquery to call web services[
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how to call webservice using jquery[
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Consuming Webservice[
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How to call a Web Service from client-side JavaScript using ASP.NET AJAX[
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