How to POST to HTTPs?

you can change the form's action with javascript:

var form = document.getElementById("post-form");
form.action = location.href.replace(/^http:/, 'https:');

But there are some security considerations, and I would suggest you to redirect your form's url to https. And although you could do it from javascript, you should never trust javascript when it gets to security, so do it from the server (it's also faster, the page doesn't have to be loaded, only the http header)