You can't do this with HTML or JavaScript - or Formspree. If you use a server-side language (i.e. ASP.NET or PHP) as well, you better set the "action" to something on your own server and read the email address from there and send an email there - but as you use formspree, I don't think you use a server-side language.
However, there are alternatives if you don't use a server-side language:
- Use an email client that supports scripting, write a script that handles incoming email from the form, extracts the email address, and replies. Your computer and client would have to be turned on all the time though, otherwise you wouldn't have instant replies.
- If you don't want to use a mail client, you can pick a programming language to write some software that communicates with the email server of your email address (look into the IMAP or POP3 protocols) and process the emails like this. This is of course harder than email client scripting, because you also have to deal with the email server communication. And again, you need to have this running 24/7 to guarantee instant replies.