I'm trying to pass a unique identifier from Marketo when a user clicks on a survey URL in an email. It seems like there are multiple unique identifiers in Marketo, but I am looking for one that I can use to track back to the user's email address in the system. So, I should be able to use the unique identifier to actually look that person up in Marketo. So far, I see Marketo Unique Code, Marketo Unique Id and Lead Id. What is the difference between all of these? When I look at the Id field in the Lead database it is a series of 6-8 numbers. However when I click on that Id and look at the user record, it no longer shows the id, it shows a field called Marketo Unique Code which is a combination of letters and numbers. If I then go back and do a search based on the Marketo Unique Code, I get zero results. Which code should I use to pass through a URL AND look up a user, and what is the token name?