Using API to create incident

I am trying to use API in order to create an incident within On-Demand Service Desk. I have created simple HTML with form element and a submit button. This is the attached file submit_form.txt and it works well. Incident is created with parameters specified in INPUT_DATA element - as it is described in On-Demand WIKI.

That simple HTML does not offer returning a proper response to user, so one needs to use JavaScript and XMLHttpRequest. Attached file submit_js.txt is showing the same form, but this time submit is carried out by JavaScript function so response can be presented to the user. Every parameter is the same, but this time incident is not created. Response from the server is this:

{"operation":{"result":{"status":"Failed","message":"Invalid api URL (or) incorrect value for OPERATION_NAME","error_code":4001,"input_field":"OPERATION_NAME"}}}

Any help is appreciated - thank you