Making Button Disabled WITHOUT Using OnClick?

When a button is clicked, the button then disables (as well as running its normal function).Obviously I'm used to using this.disabled=true but in this instance I cannot access the onClick in the <input> tag because the page is hard coded by a third party application.So I'm trying to make a function which locates the button, notices if it's been clicked and disables it on click. Seems easy to me! However I cannot make it work!Here are the two variations I've tried (both call the function in the page onLoad):

function disableButton() {
document.getElementById('submit').onclick=document.getElementById('submit').disabled=true;
}

[code]....