The "thankyou" page can only be showed if the page is in the root

I have tried as in the tutorial described the Form page example width the "thankyou" page. It doesn't worked if the site was under the parent form site. The error "RuntimeException (400)
Display page not found. Please check the page exists" was shown.
If I moved the "thankyou" site to the root then it worked.

My question is if it is configurable in a config site?

It's known to work, but must be a subpage of the form. So if the form is under /form, use /form/thankyou/ as a container folder for the thankyou page, and write - display: thankyou

Had the same issue, works if you write - display: /form/thankyou

It helps, thank you!