My Tampermonkey script works but Tumblr refills the original information

Question

I'm working on a script to remove certain pre-loaded values in my tumblr blog when I share something.

What it does right now is click on the settings icon

Then it removes the content source:

The problem is that after running the script, and submitting the post, the Content source is still appearing... seems Tumblr detects this and refills the value.

This doesn't happen if I do it manually.

Any clue on why this is happening?

I even put the setTimeout function and clicked on it to try to avoid it but it's not "saving" the information after.

// ==UserScript==
// @name         RemoveContent
// @namespace    http://www.myblog.com
// @version      0.1
// @description  Removes pre-loaded stuff
// @author       You
// @match        https://www.tumblr.com/*
// @require  http://ajax.googleapis.com/ajax/libs/jquery/1.8.3/jquery.min.js
// @require  https://gist.github.com/raw/2625891/waitForKeyElements.js
// @grant    GM_addStyle
// ==/UserScript==

document.querySelector(".post-settings").click();

waitForKeyElements (
    "#sourceUrl_input",
    changeContentInput
);

function changeContentInput () {
    setTimeout(function(){ document.getElementById('sourceUrl_input').click(); }, 1000);
    setTimeout(function(){ document.getElementById('sourceUrl_input').value = ""; }, 1000);
    setTimeout(function(){ document.getElementById('sourceUrl_input').click(); }, 1000);
}