How can I make a code appear on the header of just one page?

If you have access to WordPress files, you can add this code to your theme’s functions.php file:

function add_code_to_specific_page() {
    if ( is_page( 4315 ) ) { // The ID of your page is 4315
        ?>
        <!-- Your code here -->
        <script>
            !function (c, h, i, m, p) {
                m = c.createElement(h), p = c.getElementsByTagName(h)[0], m.async = 1, m.src = i, p.parentNode.insertBefore(m, p)
            }(document, "script", "https://chimpstatic.com/mcjs-connected/js/users/db2e7f5a6f13c1ca0590c5030/21a00c59167e1cc5c1847c037.js");
        </script>
        <?php
    }
}

add_action( 'wp_head', 'add_code_to_specific_page' );

If you don’t have access to your theme’s functions.php file, you can install the Insert Headers And Footers plugin, and add the following code to your header using that plugin:

if ( is_page( 4315 ) ) { // The ID of your page is 4315
    ?>
    <!-- Your code here -->
    <script>
        !function (c, h, i, m, p) {
            m = c.createElement(h), p = c.getElementsByTagName(h)[0], m.async = 1, m.src = i, p.parentNode.insertBefore(m, p)
        }(document, "script", "https://chimpstatic.com/mcjs-connected/js/users/db2e7f5a6f13c1ca0590c5030/21a00c59167e1cc5c1847c037.js");
    </script>
    <?php
}