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*** NOTE: As Sanford pointed out in his post below, my code below has flaws and should be improved upon.I offer this only as an explanation of what worked for my purposes at the time. My team ended up not needing to capture this info, so I never went back and updated my code. Use it at your own risk. 
***
I scoured these forums as well as myriad other sites and pages for weeks to figure out how to capture the original referral source of a user’s first visit, even if they don't convert at that time. But I never found a complete solution I could implement since I am not an experienced developer. Therefore, in order to save others from the frustration I experienced, I offer the following detailed explanation of what finally worked for me. Specifically, this is how I was able to set the values from a URL with UTM parameters as cookies so a visitor’s original referral source could be captured.
For example, a new user comes to a demo signup form on a Marketo landing page from a LinkedIn ad containing UTM parameters. He leaves the page without filling out the form and visits our company’s main website. He then later returns and completes a whitepaper request form. We want to know that he originally came from the LinkedIn ad.
Alternately, if you are sending users to your main website and want to pass UTM values to a Marketo form once they land on a Marketo landing page, uncomment the following line in the code. (Don't forget to change "example.com" to your domain.)
document.cookie = key + "=" + query[key] + ";domain=.example.com;path=/";
Put the code below on your form pages. (I know there are more efficient ways of writing it, but I don't have the Javascript skills to do so.) You must have hidden fields on the forms to capture the UTM parameters. When setting up the hidden fields on the form be sure to set Get Value from in the Autofill settings as “Cookie Value”.
In this example I have “utm_source”, “utm_campaign”, “utm_medium”, “utm_term”, and “utm_content” as hidden fields on my form.
Put the following in the <head> of your page:
<script type="text/javascript">
//This grabs the UTM parameters from the URL
function getQueryParams(qs) {
qs = qs.split("+").join(" ");
var params = {}, tokens,
re = /[?&]?([^=]+)=([^&]*)/g;
while (tokens = re.exec(qs)) {
params[decodeURIComponent(tokens[1])] = decodeURIComponent(tokens[2]);
}
return params;
}
//This sets the cookies
var query = getQueryParams(document.location.search);
for (var key in query) {
//console.log(key, query[key]);
document.cookie = key + "=" + query[key];
//Setting domain and path attributes allow cookies to be read across subdomains
//To have a cookie available to all subdomains, you must put a . in front of your domain.
//Setting the path=/ will have the cookie be available within the entire specified domain.
//Don't forget to change "example.com" to your domain.
//document.cookie = key + "=" + query[key] + ";domain=.example.com;path=/";
}
//This gets the cookies
//ID = MARKETO FIELD NAME
var elem = document.getElementById("utm_source");
var elem2 = document.getElementById("utm_campaign");
var elem3 = document.getElementById("utm_medium");
var elem4 = document.getElementById("utm_term");
var elem5 = document.getElementById("utm_content");
elem.value = query.utm_source;
elem2.value = query.utm_campaign;
elem3.value = query.utm_medium;
elem4.value = query.utm_term;
elem5.value = query.utm_content;
//This runs the script when the page loads so the hidden fields are automatically populated
window.onload = getQueryParams;
</script>
Hi Rob! Nice work there, but some red flags too... 
You shouldn't use a timer/interval to integrate with Marketo forms, as they have their own robust event model, and similarly you don't need to manage your own hidden input elements... document.hasOwnProperty('cookie') doesn't really tell you anything about cookie support, and the domains setting doesn't seem to be used... and params[] = <value> is bad JS syntax (are you trying to append to the hash PHP-style?).
At any rate, I advise anyone interested in this area to check out ConversionPath (http://conversionpath.io/). I wrote the JS client piece, and it has seamless integration into the Marketo forms event model (as well as an integration into 3rd-party forms that I think is unprecedented) , a bunch of configurable bells and whistles, and is under continuous development.
Beth -
I am currently using the code shown in my original post. However, as Sanford pointed out, it has its flaws, so use at your own risk.
Hi Sanford -
Yes, I was waiting for your response! In fact, I almost mentioned in the post that you could probably write the code much better and answer any questions that might arise much better than I could, but didn't want to put you on the spot like that.
I actually received the code from Marketo's Lead Technical Consultant and tweaked a few things. From all the posts I read on Stack Exchange I figured I needed a different regex, but couldn’t get figure out how to get anything else to work. For what it's worth, here's the exact code I received from Marketo, in its entirety:
<html>
<p>
// fetch url name value parameters and can create cookie
// url?cookieName=cookieValue
</p>
<script src="//app-abk.marketo.com/js/forms2/js/forms2.min.js"></script>
<form id="mktoForm_1071"></form>
<script>MktoForms2.loadForm("//app-abk.marketo.com", "898-UZL-601", 1071);</script>
<script>
function getQueryParams(qs) {
qs = qs.split("+").join(" ");
var params = {}, tokens,
re = /[?&]?([^=]+)=([^&]*)/g;
while (tokens = re.exec(qs)) {
params[decodeURIComponent(tokens[1])] = decodeURIComponent(tokens[2]);
}
return params;
}
function btnClickFunction() {
var query = getQueryParams(document.location.search);
for (var key in query) {
console.log(key, query[key]);
document.cookie = key + "=" + query[key];
}
var elem = document.getElementById("customCookieField");
elem.value = query["cookieName"];
}
</script>
<button onClick="btnClickFunction()"> set cookie </button>
</html>
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