Hi everyone! I'm working on a project in which we are trying to track what site a lead was on before they got to our site. For example, if the lead comes from a certain nonprofit's site, we can react differently to them. I've been trying to track off GA but it's proven to be a bit difficult. We also tried to do this through RTP and it doesn't work. Has anyone done anything like this before? Or does anyone have any tips on ways to capture a lead's referral information?
If a site passes the Original Referrer info and it is a new lead, you can check that information.
You can also use Visits Web Page: Referrer
But this isn't 100% reliable and even if you did Ad tracking with UTMs, it may not always work either. But there could be solutions for this, so check launchpoint.
There are two critical components here:
 Capturing the source without using source-tagged URLs (i.e. UTM tags) means relying on the HTTP(S) referrer. For greatest accuracy, your LPs must be accessed over https://. If the non-profit in question is now using https://, or ever decides to in the future, you must also be doing so, or you'll never see the info you need.
 Persisting (saving) the original source info in a cookie(s) so it can be accessed at the point of conversion, typically by adding hidden fields to a form. Marketo Forms can automatically populate fields from cookie values, but saving to the cookie is done using your own JS code.
As Josh mentions, the Original Referrer field will be set to the referrer (the full URL) of the lead's first recorded web activity (provided if, as in  above, it is available in the browser). While it's nice additional convenience, you'll probably want to parse out individual touch elements (Original Source/Medium/Channel/etc.) and use friendly names (LinkedIn, Google, SomeBlog) rather than hostnames.
speaking of HTTPS. Anyone knows if its possible to run HTTPS Marketo LPs with the CNAME Domain instead of Marketo as the TLS certificate holder? ( Well this is not technically correct but you get what I mean, right? ). Is there a product package that includes this?
There's no Marketo level that includes it for free AFAIK; you always have to pay an onboarding fee, plus a not-insignificant yearly fee, in addition to your standard subscription. (In addition to renewing your cert every year/2 years, though you're likely doing this for your main site already.)
The alternative is to route all your LP traffic via a CDN, which typically will have no onboarding or yearly fees at all for SSL.
I use the referrer constraint on the visits web page filter. For Example: Smart list contains that filter Visits page: Value www.homepage.com with constraint referrer contains <domain name>