Question in title, but I’ll elaborate.
Say I have a form on a non-secure page, but I don’t want the data that the user is posting to my web server to make sense to anyone who might intercept it. Do I need to serve the form securely or simply post the form to a secure URL?
By serving the form unsecured, you allow a man-in-the-middle to alter your form’s POST destination, letting an attacker harvest login information. But MITM attacks are not common, so you’re probably fine serving the form unsecured.