jQuery .trigger()
is mostly only good for events set by jQuery. And, in this case, it must be called using the target page's instance of jQuery, not the userscript's. (Also, how is your script using jQuery? The wrong method can bust the page, the script, or both.)
There may be other events required and/or AJAX to contend with, we can't tell from your question. See Choosing and activating the right controls on an AJAX-driven site.
Polling for the node and triggering mouse event(s) will work in almost all cases. Here's a complete userscript to get you started:
// ==UserScript==
// @name _Auto-click the sign-in link
// @include http://YOUR_SERVER.COM/YOUR_PATH/*
// @grant GM_addStyle
// ==/UserScript==
var signInTimer = setInterval ( function() {
var signInBtn = document.querySelector ("#signInBtn");
if (signInBtn) {
clearInterval (signInTimer);
var clickEvent = document.createEvent ('MouseEvents');
clickEvent.initEvent ('click', true, true);
signInBtn.dispatchEvent (clickEvent);
}
}
, 200
);