To use Formspree with Dreamhost Remixer, first create an account on Formspree, then create a new project and create a new form in the project.
Copy the part of the endpoint after formspree.io found in the integration tab of the form's settings shown under "Your form's endpoint is:". For example /f/your-form-id
and paste this into the "Contact Form Email Address " field in Remixer.