How to use formspree redirect like explained in the ghost official documentation

Hello,

I am reading Ghost Docs and it seems possible to create a contact form and customize the success page:

It seems that this feature is now no more available, see "Thank You" redirect – Formspree Help for more information.

I just tested and I confirm this is not possible anymore with the free plan.

Since we already configured mailgun for sending ghost emails, is there a way to develop a plugin that exposes an endpoint for posting email?