how to prefill post content


I got a button which will appear in all posts. I used this snippet (for testing) in the functions.php but it has no effect.

    
    add_filter( 'default_title', function() {
        return 'My default title';
    });

    add_filter( 'default_content', function() {
        return 'My default content';
    });

    add_filter( 'default_excerpt', function() {
        return 'My default excerpt';
    });