Hey Sarah,
To track form submissions on external landing pages, we suggest to create a Smart List with the Fills out Form filter. On that filter, the "Web Page" constraint is available so you can only gather leads who filled out a specific form on a specfic web page (an external web page or a Marketo landing page can be defined here).
The Landing Page Performance report is a report designed specifically around Marketo Landing Pages and cannot be used on external pages.
Best,
Will