WooCommerce | Set billing field value

[admin]

I want to pre-populate the values for the checkout's billing fields to the DB stored values of the user <strong>before his first purchase</strong>.

I've tried the following code:

add_filter( 'woocommerce_checkout_fields' , function ( $fields ) {
    $fields['billing']['billing_first_name']['placeholder'] = 'First Name';
    $fields['billing']['billing_first_name']['default'] = wp_get_current_user()->user_firstname;
    return $fields;
});

I've read about this solution in an <a href="https://wordpress.stackexchange.com...m-select-field-with-stored-values-on-checkout">other post</a>. Placeholder works great, but the value doesn't.

Also, the <a href="https://docs.woocommerce.com/docume...ng-checkout-fields-using-actions-and-filters/" rel="nofollow noreferrer">WooCommerce Doc</a> (Lesson 1) doesn't say about anything the array value 'default'