I'm imagining the landing page is not a Marketo landing page, right?
You can create 2 campaings, one has a trigger of lead is created the other is campaign is requested.
for the lead is created.
Under the flow action you can add: If origianal refferer is this (url of landing page) then request this campaing.
Under the requested campaign you can change lead source or have it do what ever you want allowing you to track it.
Is that what you were looking for.
You can even do that if it is a Marketo landing page. Doing it like this you have a form on your website that allowsyou to create campaigns to see where leads come from to visit your site. Such as google, bing or yahoo.