One post – split content on two pages

Cryptic title. Let me explain.

Big picture:
I’m creating a site that will have multiple pages with forms. Everytime the form submits, it should redirect to a thank you page with custom messaging depending on the form.

What I’m trying to do:
I’m trying to create ONE thank you page instead of unique pages to match each form.

In the Admin:
When I create my page with the form, I want to have a separate field “Thank You Page Content” for content for the thank you page. So on the front end, on the form page you do not see the content from this field. After filling in the form, it redirects to site.com/thankyou and will display the content from the “Thank You Page Content” custom field for the corresponding form.

I’m new to the WordPress world, with Expressionengine I could do this with URL segments. Pointers to get me started/solutions would be appreciated.