To add GNU Terry Pratchett to a write.as site, you need to include a custom HTTP header. Since write.as is a hosted platform with limited backend access, you can’t directly modify server settings. However, you can achieve the same effect using JavaScript.

1. Add a JavaScript snippet to your write.as site using the Custom JavaScript feature.
2. Insert the following script in your site’s Customize section:

(function() {
    var meta = document.createElement('meta');
    meta.httpEquiv = "X-Clacks-Overhead";
    meta.content = "GNU Terry Pratchett";
    document.getElementsByTagName('head')[0].appendChild(meta);
})();

3. Save the changes and test your site. While this won’t add a true HTTP header (since that requires server-side control), it ensures the message appears in the metadata of your page.

For the uninitiated, I first learned what this is all about, here: https://mikecoats.com/x-clacks-overhead/ & http://www.gnuterrypratchett.com