Javascript how to click a button on youtube

i'm attempting to write a very simple greasmonkey plugin for firefox (uses javascript) to click the My subscriptions button all the way to the left. The problem is this is literally the first time i've ever even looked at javascipt, and my poor attempt at the code doesn't work, could anyone help me write code to click that button

Code:

document.getElementByName('My subscriptions').onclick();