netlify-forms-react.js

 const [email, setEmail] = useState("")

 const handleSubmit = e => {
    fetch("/", {
      method: "POST",
      headers: { "Content-Type": "application/x-www-form-urlencoded" },
      body: formEncode({ "form-name": "register", email: email }),
    })
      .then(() => alert("Success!"))
      .catch(error => alert(error))

    e.preventDefault()
  }