Where to put thankyou/formdata.md in a modular page form?

I'm setting up a contact form as part of a modular page created using an Antimatter skeleton. It works: the message gets sent to the right place but the form throws an Error 400 even so, apparently because it can't find thankyou/formdata.md. I've set up thankyou/formdata.md both in the modular page folder (same directory as the modular.md file that contains the form definition) and in the _contact subfolder (same directory as the form.md file), but neither seems to be the right place. Any advice?

I've figured it out. The 'process:' bit of the form definition in modular.md must have the full path to the thankyou directory, ie '- display: /path/to/thankyou/' rather than just ' - display: thankyou'.

Discussion of the same issue:

https://muut.com/i/getgrav/general:the-thankyou-page-can-onl