Yes, that's the whole script (except I left out the PHP tags). Here it is with comments:
Start by getting your form working without using the above. Set "good_url" in your HTML form. Test until you're happy that it's working perfectly.
$_POST["good_url"] = "http://your-download-url"; // force a particular URL for a successful submission and put it in the POST data. This hides the URL form the contents of the HTML form sent to the browser.
require("formmail.php"); // call FormMail to do all its usual work
Then, when it's working correctly, do this:
- Write the above script. Call it, say, "download.php". Put the right URL in the place shown.
- Remove "good_url" from your HTML form.
- Change the "action" attribute of the FORM tag in the HTML form to be "download.php" (with path etc at the front).