Checkout custom fields – Classic checkout

add_action( 'woocommerce_before_order_notes', function( $checkout ) {

	echo '<fieldset><legend><h3>Profile information</h3></legend>';

	woocommerce_form_field(
		'secondary_fname',
		array(
			'class' => [ 'form-row-first' ],
			'type' => 'text',
			'label' => 'Secondary First Name',
			'required' => false,
		),
		$checkout->get_value( 'secondary_fname' )
	);

	woocommerce_form_field(
		'secondary_lname',
		array(
			'class' => [ 'form-row-last' ],
			'type' => 'text',
			'label' => 'Secondary Last Name',
			'required' => false,
		),
		$checkout->get_value( 'secondary_lname' )
	);

	woocommerce_form_field(
		'secondary_email',
		array(
			'class' => [ 'form-row-wide' ],
			'type' => 'email',
			'label' => 'Secondary Email Address',
			'placeholder' => '[email protected]',
			'required' => true,
		),
		$checkout->get_value( 'secondary_email' )
	);

	woocommerce_form_field(
		'licensed',
		array(
			'class' => [ 'form-row-wide' ],
			'type' => 'radio',
			'label' => 'Are you licensed?',
			'required' => true,
			'options' => [ 'yes' => 'Yes', 'no' => 'No' ],
		),
		$checkout->get_value( 'licensed' )
	);

	echo '</fieldset>';

} );

add_action( 'woocommerce_checkout_process', function() {

	$required_fields = [
		'secondary_email' => 'Secondary Email Address',
		'licensed' => 'Are you licensed?',
	];

	foreach( $required_fields as $name => $value ) {
		if( empty( $_POST[$name] ) ) {
			wc_add_notice(
				sprintf(
					'<strong>%s</strong> is a required field.',
					$value
				), 'error'
			);
		}
	}

} );

add_action( 'woocommerce_checkout_update_order_meta', function( $order_id ) {

	$order = wc_get_order( $order_id );

	$fields = [
		'secondary_fname',
		'secondary_lname',
		'secondary_email',
		'licensed',
	];

	foreach( $fields as $field ) {
		if( isset( $_POST[$field] ) ) {
			$order->update_meta_data(
				$field,
				sanitize_text_field( $_POST[$field] )
			);
		}
	}
	$order->save();

} );

Specifications here: https://rudrastyh.com/woocommerce/woocommerce_form_field.html