Create Custom Billing Field and pre fill Auto fill checkout default filed and custom filed from user data saved in Woocommerce session

functions.php

/**
 * C.1. Custom Billing Field
 */
 add_filter( 'woocommerce_billing_fields' , 'wc_custom_fields' );

 function wc_custom_fields( $fields ) {
      $fields['billing_reservationcode'] = array(
 		'type'          => 'text',
 		'required'		=> true,
		'class'         => array('reservationcode'),
 		'label'         => 'Reservation Code',
		'label_class'   => 'reservation-code'
 	 );
   return $fields;
 }

add_action( 'template_redirect', 'set_custom_data_wc_session' );
function set_custom_data_wc_session () {

    if ( isset( $_GET['fname'] ) ) {
        $fname   = isset( $_GET['fname'] )   ? esc_attr( $_GET['fname'] )   : '';
    } 
    if (isset( $_GET['lname'] )) {
    
        $lname = isset( $_GET['lname'] ) ? esc_attr( $_GET['lname'] ) : '';
    }
    if (isset( $_GET['email'] )) {
  
        $email = isset( $_GET['email'] ) ? esc_attr( $_GET['email'] ) : '';
    }
    if (isset( $_GET['rcode'] )) {
  
        $rcode = isset( $_GET['rcode'] ) ? esc_attr( $_GET['rcode'] ) : '';
    }


        // Set the session data
        WC()->session->set( 'custom_data', array( 'fname' => $fname, 'lname' => $lname, 'email' => $email, 'rcode' => $rcode) );
    }

// Autofill checkout fields from user data saved in Woocommerce session
add_filter( 'woocommerce_billing_fields' , 'prefill_billing_fields', 99999 );
function prefill_billing_fields ( $address_fields ) {
    // Get the session data
    $data = WC()->session->get('custom_data');
	// First Name
    if( isset($data['fname']) && ! empty($data['fname']) )
        $address_fields['billing_first_name']['default'] = $data['fname'];
    // Last Name
    if( isset($data['lname']) && ! empty($data['lname']) )
        $address_fields['billing_last_name']['default'] = $data['lname'];
    //var_dump( $data['lname']);exit;
    // Email
    if( isset($data['email']) && ! empty($data['email']) )
        $address_fields['billing_email']['default'] = $data['email'];
    // reservationcode
    if( isset($data['rcode']) && ! empty($data['rcode']) )
         $address_fields['billing_reservationcode']['default'] = $data['rcode'];
     // echo'<pre>';
     // var_dump($address_fields);

    return $address_fields;
}