Submitting automatically a form with JavaScript

Hello,
I want to create a page (HTML/PHP) that loads an URL, in this URL there are a submit button that doesn't have a name but have an id ("u721957_1"). I want that when I load this page, automatically this submit button is pushed. I don't like to submit the form directly because there are some data in the button that I need, I tried this: document.getElementById('u721957_3').click() as a function in the onLoad of the body but it doesn't work (you have to note that I can't change the id of the button).
Any solution please??
