Add custom field to prepopulation list

functions.php

<?php

function lw_woocommerce_gpf_custom_field_list( $list ) {
    // Register the _my_custom_field meta field as a pre-population option.
    $list['meta:_my_custom_field'] = 'My custom field';
    return $list;
}
add_filter( 'woocommerce_gpf_custom_field_list', 'lw_woocommerce_gpf_custom_field_list' );