pewc_after_create_product_extra.php

<?php
/**
 * Trigger an action when an add-on field is present in an order
 * Note that you will need to change the values for $field_id and $value below
 */
function prefix_pewc_after_create_product_extra( $add_on_id, $order, $field ) {

	$field_id 	= 1234;
	$value 		= '__checked__';
	// Check specific field ID is checked
	if( $field['field_id'] == $field_id && $field['value'] == $value ) {
	// Do our action here
		prefix_send_custom_email();
	}

}
add_action( 'pewc_after_create_product_extra', 'prefix_pewc_after_create_product_extra', 10, 3 );

 /**
 * Send an email when an add-on field is present in an order
 * You'll need to change the values for $to, $subject and $content below
 */
function prefix_send_custom_email() {

	$to 		= 'you@youremail.com';
	$subject 	= 'Your Subject Line';
	$content	= 'Add your content here.';

	ob_start();
	wc_get_template( 'emails/email-header.php', array( 'email_heading' => $subject ) );
	$email = ob_get_clean();
	$email .= $content;
	wc_get_template( 'emails/email-footer.php' );
	$email .= ob_get_clean();

	wc_mail( $to, $subject, $email );

}