Automatic click using javascript on webpage

I am currently experimenting something, and I became to have to click a button on a website which I didn't make, automatically using files made up of javascript and other web languages.

I read this one: How do I programmatically click a link with javascript?

But if I want to auto-click a website which I didn't make, so I can't get access to the website's files. Then, should (or can) I load the website's .php or .html and attach my snippets which contain document.getElementById('yourLinkID').click(); ?

If I can load the website's files, and attach my code and make it run, How can I do that? Or is there other possible ways?

Since I try to click a site whose files aren't accessible, I can't figure out how can I click button