The way to usually do it, is combine normal form submission with
GET (the server accepts the form submission, then redirects the page).
With normal form submission, all you need to do is mark the fields as
required like so:
<form method="post" action="/what/ever"> <label>Name: <input required></label> <label>Email: <input type="email" required></label> <input type="submit" value="Send"> </form>
After the form submission is successful, redirect the page from the server-side to the next page (or to an error page, in case something went wrong).