Analysing Javascript from other page to emulate POST request

There is a page with a form. When I press the "send" button, two values next.x and next.y are added to the POST request.

How do I find out where these values are coming from? Without those values the POST request is not accepted by the page.

I 开发者_运维知识库searched all files but I could not find any next.x anywhere. I tried different javascript debuggers and there is a function triggered when the submit button is pressed. But I could not find those values there either.

Also after the script is executed the values still not in the form. I checked input fields and target url.

This is because you're using an input with type="image", a sort of "graphical submit control", to submit the form. Change it to a normal <input type="submit"..., style it with CSS, and you'll be getting what you want.

Read up on what the spec says here