[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' );

You should be able to use the e.g. first option as the default like this, so you would not hard code the value in the hook:

$fields['billing']['test_radio']['default'] = array_values($fields['billing']['test_radio']['options'])[0];

You should be able to use the e.g. first option as the default like this, so you would not hard code the value in the hook:

$fields['billing']['test_radio']['default'] = array_values($fields['billing']['test_radio']['options'])[0];

strangely this doesn't work for additional fields ie.:
$fields['additional']['test_radio']['default'] = array_values($fields['additional']['test_radio']['options'])[0];
or
$fields['additional']['test_radio']['default'] = 'one';

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 );