Use the appropriate constraint to specify which page the form is on when using a "Fills Out Form" trigger or "Filled Out Form" filter in a Smart List. There are three options:
Web Page
This constraint is designed for Marketo Landing Pages. If you have one form on multiple pages, and were attempting to specify a Marketo Landing Page, then you would use Web Page. If you use Web Page, your value is the Landing Page asset name, such as "Global Contact Form Page". Or if the page is local to a program, the name format would be [program name].[landing page name]
Referrer URL
This constraint is designed for external, non-Marketo pages. If the form you have is embedded on an external page, the value you would enter would be the URL of the page.This constraint is very picky about the values entered. Say the form is on http://www.pages.domain.com/contact-us.html. Only the following 2 options would function:
By using "is", the exact value must match. When using "contains" a smaller portion of the whole can be used. Even "contains" /contact-us would have worked successfully, but the less specific the value is the more other pages could qualify depending on the use case. Generally, "contains" is advised with a unique portion of the URL because there can often be querystrings on URLs that would then not satisfy the "is" constraint.
Querystrings
If the form is on a page, and the page has a querystring, such as: http://www.pages.domain.com/contact-us.html?product=new-stuff, then the form submission activity on the Marketo record would have a new field in the Detail called Query Parameters. (Querystring, Query String, Query Parameters are all the same thing.) If you set up direct links to the Page with unique querystrings, then you would be able to use the Querystring constraint and use values from the querystring in the trigger/filter form fillout campaign.