How to get custom field value in observer from checkout custom form

hey there here i am trying to save the values my custom checkout form’s values in db tables .

Here is my custom form.

i have used event observer to save values and hard coded values are getting save but i am not able to get dynamic values.

public function execute(MagentoFrameworkEventObserver $observer) {
    $order = $observer->getEvent()->getOrder();
    $quote = $observer->getEvent()->getQuote();

    $order->setData('custom_firstname', 'Custom Checkout Form');
    $quote->setData('custom_firstname', 'Custom Checkout Form');
}

After place order “Custom Checkout Form” is properly saving in the table but i want my form’s first name value and other should also save.
How can i do this is anyone has idea how can i save them also.
Thanks in Advance 🙂