Custom fields in pages

Hi,

thoughts and feedback on the following would be much appreciated.

My intention:

I have 3 external webpages which contain links to a form on my wordpress website. After the form is submitted a reference Id should be passed a long with this form to an external database.

What I tried doing:

First I tried experimenting with query_var so I pass the refID in the url to the form webpage and on submit, the refID is passed along with other form data, using the custom plugin along a url to an external database, however I did not manage.

What I’m thinking of:

I replicate the forms page 3 times, where each page has a custom field with a different value and each of the external links is directed to 3 different form pages (which are the same in reality) but have different custom field values. The forms page which then calls the custom plugin that generates the url that populates the database with the form data, should also get the custom field value depending on the page it was directed to. Is this possible? Can I retrieve the custom field value in the plugin php file?

Thanks