Example plugin to test proposed `woocommerce_form_field_value` filter

woocommerce-form-field-value.php

<?php
/**
 * Plugin Name: WooCommerce Form Field Value Example
 * Author: Michael Bragg <email@michaelbragg.com>
 * Version:     1.0.0
 */

function custom_checkout_field( $checkout ) {

	echo '<section id="example-field">';

	// Echo the checkbox field with label text.
	woocommerce_form_field( 'example_-field', array(
		'type'      => 'checkbox',
		'class'     => array( 'input-checkbox' ),
		'label'     => esc_html( 'Example of checkbox field.' ),
		'required'  => false,
	), $checkout->get_value( 'example_field' ) );

	echo '</section>';

}

add_action(
	'woocommerce_after_order_notes',
	'custom_checkout_field'
);

/**
 * Test the filter is working.
 */
function custom_checkout_field_checked( $value, $key, $args ) {

	if ( 'example_-field' !== $key ) {
		return $value;
	}

	$value = 1;

	return $value;

}

add_filter(
	'woocommerce_form_field_value',
	'custom_checkout_field_checked',
	10,
	3
);