Add a custom option page to WordPress Options

<?php
// Add settings page for setting parliamentary dossier image on frontpage
add_action('admin_menu', 'register_parliamentary_dossier_settings_page');

function register_parliamentary_dossier_settings_page() {
    add_submenu_page(
        'options-general.php',
        __('Parliamentary Dossier Settings', 'astra-child'),
        __('Parliamentary Dossier', 'astra-child'),
        'manage_options',
        'media-selector',
        'parliamentary_dossier_settings_page_callback'
    );
}

// Settings page callback
function parliamentary_dossier_settings_page_callback() {

    // Save attachment ID
    if (
        isset($_POST['submit_image_selector']) &&
        isset($_POST['image_attachment_id'])
    ) {
        update_option(
            'parliamentary_dossier_attachment_id',
            absint($_POST['image_attachment_id'])
        );
    }

    wp_enqueue_media();

    printf(
        '<h1>%1$s</h1>
        <h2 style="font-size: 20px;">%2$s</h2>
        <form method="post">
            <div class="image-preview-wrapper" style="margin-bottom:15px;">
                <img id="image-preview" src="%3$s" height="400">
            </div>
            <input id="upload_image_button" type="button" class="button" value="%4$s" />
            <input type="hidden" name="image_attachment_id" id="image_attachment_id" value="%5$s">
            <input type="submit" name="submit_image_selector" value="%6$s" class="button-primary">
            <p>%7$s</p>
        </form>',
        esc_html__('Parliamentary Dossier', 'astra-child'),
        esc_html__('Frontpage Image', 'astra-child'),
        esc_html(wp_get_attachment_url(get_option('parliamentary_dossier_attachment_id'))),
        esc_html__('Upload image', 'astra-child'),
        esc_html(get_option('parliamentary_dossier_attachment_id')),
        esc_html__('Save', 'astra-child'),
        esc_html__('Recommended resolution: 356x659.', 'astra-child')
    );
}

// Load media uploader script
add_action('admin_footer', 'parliamentary_dossier_print_scripts');

function parliamentary_dossier_print_scripts() {

    $my_saved_attachment_post_id = get_option('parliamentary_dossier_attachment_id', 0);
    ?>
    <script type="text/javascript">
        jQuery(document).ready(function ($) {

            var file_frame;
            var wp_media_post_id = wp.media.model.settings.post.id;
            var set_to_post_id = <?php echo (int) $my_saved_attachment_post_id; ?>;

            $('#upload_image_button').on('click', function (event) {
                event.preventDefault();

                if (file_frame) {
                    file_frame.uploader.uploader.param('post_id', set_to_post_id);
                    file_frame.open();
                    return;
                }

                wp.media.model.settings.post.id = set_to_post_id;

                file_frame = wp.media.frames.file_frame = wp.media({
                    title: 'Select an image to upload',
                    button: {
                        text: 'Use this image'
                    },
                    multiple: false
                });

                file_frame.on('select', function () {
                    var attachment = file_frame.state().get('selection').first().toJSON();

                    $('#image-preview').attr('src', attachment.url).css('width', 'auto');
                    $('#image_attachment_id').val(attachment.id);

                    wp.media.model.settings.post.id = wp_media_post_id;
                });

                file_frame.open();
            });

            $('a.add_media').on('click', function () {
                wp.media.model.settings.post.id = wp_media_post_id;
            });

        });
    </script>
    <?php
}