[Checkout Field Editor] Setting a default value for a radio button field

wc-set-default-radio-field.php

function custom_override_checkout_fields ( $fields ) {
	$fields['billing']['test_radio']['default'] = 'one';
	return $fields;
} // End custom_override_checkout_fields()

add_filter( 'woocommerce_checkout_fields' , 'custom_override_checkout_fields' );

Same for me, can't get it to work with Additional Fields.

For anyone else that finds themselves here with the same issue. I found that the WooCommerce Checkout Field Editor plugin set a priority of 1000 for their add_filter. So in order to customize things in additional (which is actually order) change the snippet to this which worked for me:

function custom_override_checkout_fields ( $fields ) {
	$fields['order']['email_optin']['default'] = 'Yes';
	return $fields;
} // End custom_override_checkout_fields()

add_filter( 'woocommerce_checkout_fields' , 'custom_override_checkout_fields' , 1100 );