In Fluent Forms, set the values of a checkbox to a post type's posts

fluent-forms-post-type-checkbox.php

<?php

add_filter('fluentform/rendering_field_data_input_checkbox', function ($data, $form) {
    if ($form->id != 1) {  // Form ID
        return $data;
    }

    // Set which field to do this on
    if (\FluentForm\Framework\Helpers\ArrayHelper::get($data, 'attributes.name') != 'products') { // Field name
        return $data;
    }

    // Query for products
    $args = [
        'post_type'      => 'product',
        'posts_per_page' => -1,
        'post_status'    => 'publish'
    ];

    $query = new WP_Query($args);
    $options = [];

    if ($query->have_posts()) {
        while ($query->have_posts()) {
            $query->the_post();
            $options[] = [
                'label'      => get_the_title(),
                'value'      => get_the_ID()
            ];
        }
        wp_reset_postdata();
    }

    // Clear existing options before adding new ones
    $data['settings']['advanced_options'] = [];

    // Add new options
    if (!empty($options)) {
        $data['settings']['advanced_options'] = $options;
    }

    return $data;
}, 10, 2);