Need help with Click()

I'm new to JS and i'm trying to write a script that will do something like this:

If on page #1
//click button1
else if on page#2
//click button2
else
//click button3

To click a button i use:
document.Form_Name.Button_Name.click();

It works fine as long as i know the form name and the button name like clicking the search button in google would be:
document.f.btnG.click();

But some forms don't have a name. All they have is something like:
<form action="Link" method="post" onsubmit="this.submit.disabled=true;">
.
.
.
and the button is:
<input name="submit" type="submit" onload="this.disabled=false;" value="OK">


How can i click such a button?

document.forms[0].buttonName.click();

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It worked. Thanks.