There is actually a token for this:
I have not used it, so I am assuming that it will give you the url, but there is a chance that it will give you the name of the Mkto landing page if the form is on a Mkto landing page and not used in an iframe. As always, make sure to test!
Here is an article for your reference: https://community.marketo.com/MarketoArticle?id=kA050000000KywJ