Permalinks Question: Adding a prefix ONLY in front of the posts

1) Add this rewrite at the end of you function.php

function add_rewrite_rules( $wp_rewrite )
{
    $new_rules = array(
        'YOUR_PREFIX/(.+?)/?$' => 'index.php?post_type=post&name=". $wp_rewrite->preg_index(1),
    );

    $wp_rewrite->rules = $new_rules + $wp_rewrite->rules;
}
add_action("generate_rewrite_rules', 'add_rewrite_rules'); 

function change_blog_links($post_link, $id=0){

    $post = get_post($id);

    if( is_object($post) && $post->post_type == 'post'){
        return home_url('/YOUR_PREFIX/'. $post->post_name."https://wordpress.stackexchange.com/");
    }

    return $post_link;
}
add_filter('post_link', 'change_blog_links', 1, 3);

2) Go to Settings > Permalinks and click Save Changes.