How can I click a button on other's website by code?

I am finding way to click on webpage which isn't mine, automatically, under certain circumstances, using codes like

document.getElementByid('yourLinkID').click();

Problem is that, since I don't own the webpage, I can't upload a file which contains (javascript) code, so I can't figure out how to apply the code to actually click the button. By how can I make the code actually work on other's website?

Is there any way I can accomplish this?