How do I make a form button disabled until form is filled in?

First, add disabled=”disabled” to your submit button:

Second, you’ll need to add some JS to check that each field is filled out before removing the disabled tag:

(function() {
    var inputs = $('form input');
    var button = $('#submit-btn');

    inputs.keyup(function() {

        var empty = false;
        inputs.each(function() {
            if ($(this).val() == '') {
                empty = true;
            }
        });

        if (empty) {
            button.attr('disabled', 'disabled');
        } else {
            button.removeAttr('disabled'); 
        }
    });
})()