react-hook-form validation with form POST i.e. not XHR

I have an existing react app that contains various forms, all using react-hook-form (v6) to validate the inputs prior to submission.

I’m creating a new form that also requires client side validation provided by react-hook-form, however, the form needs to work using a regular HTML for POST, i.e. not an XHR request, an actual POST to a url.

My issue is that the form validation never seems to run. As I said, I have a number of other forms all using react-hook-form that work just fine.

If anyone has come accross this same problem or has a solution it would be much appreciated. Thanks.