Automatically add custom taxonomy to posts in a category

You’ll want to hook onto the wp_insert_post action. Once the new post is created, you can then assign it to the taxonomy/term you wish.

It might look something like this:

add_action('wp_insert_post', function($postId, $post, $updatingExisting) {
    if( $updateExisting )
        return;

    // read up on these params at http://developer.wordpress.org/reference/functions/wp_set_post_terms/
    wp_set_post_terms( $postId, 'main_podcast', 'qtserie', true);
}, 10, 3);

Depending on what else is happening on your site, you might need to do some checking inside the filter to make sure you’re only updating the posts you want to (this will run on creation of all posts, pages, custom-post-types, etc.