Gravity Forms User Reg Add-on Prepopulate

snippet.php

<?php

add_filter(
    'gform_user_registration_user_data_pre_populate',
    function( $mapped_fields, $form, $feed ) {

        // Select field - this works
        $mapped_fields[5] = 'input_value';

        // Checkboxes field - does not work
        $mapped_fields[8] = 'input_value';

        return $mapped_fields;
    },
    10,
    3
);