Handler for the "onkeydown" event to submit the autocomplete input by hitting the Enter key, also input must have a 'auto-submit' class

autocomplete.custom.js

(function ($) {
  Drupal.jsAC.prototype.onkeydown = function (input, e) {
    if (!e) {
      e = window.event;
    }
    switch (e.keyCode) {
      case 13: // Enter.
        if ($(input).hasClass('auto-submit')) {
          this.hidePopup(e.keyCode);
          input.form.submit();
        }
        return true;
      case 40: // down arrow.
        this.selectDown();
        return false;
      case 38: // up arrow.
        this.selectUp();
        return false;
      default: // All other keys.
        return true;
    }
  };
})(jQuery);