Set default taxonomies to custom post types in WordPress

gistfile1.txt

Usually for normal posts, if a user doesn't select a custom category then it defaults to 'uncategorized', with custom post_types
that isn't always the case. If you want to have your custom post_type default on a certain taxonomy you can add this function
to your functions.php file in your template directory. 

/**
 * Set default cat for cpt
 * @source {https://circlecube.com/says/2013/01/set-default-terms-for-your-custom-taxonomy-default/}
 * @source {http://wordpress.mfields.org/2010/set-default-terms-for-your-custom-taxonomies-in-wordpress-3-0/}
 * @license   GPLv2
 */
function set_default_object_terms_203962( $post_id, $post ) {
if ( 'publish' === $post->post_status && $post->post_type === 'your_post_type' ) {
    $defaults = array(
        //'your_taxonomy_id' => array( 'your_term_slug', 'your_term_slug' )
        'post_tag' => array( 'taco', 'banana' ),
        'monkey-faces' => array( 'see-no-evil' ),
        );
    $taxonomies = get_object_taxonomies( $post->post_type );
    foreach ( (array) $taxonomies as $taxonomy ) {
        $terms = wp_get_post_terms( $post_id, $taxonomy );
        if ( empty( $terms ) && array_key_exists( $taxonomy, $defaults ) ) {
            wp_set_object_terms( $post_id, $defaults[$taxonomy], $taxonomy );
        }
    }}
}
add_action( 'save_post', 'set_default_object_terms_203962', 100, 2 );

Here you just need to replace 'your_post_type' with the type of custom post you want to set the default on and then switch out 
'your_taxonomy_id' and 'your_term_slug' with the property values. You can find these by going to that taxonomy in your WordPress
(so if I'm wanting a category, I'd click on my categories link in my side bar) then you can look in your url to find the 
taxonomy id, which is probably just the name of it. Then you slug should be in one of the columns under your individual
taxonomy types. 

After you have saved this, it won't work on posts already created, but will work on any new posts from then on out. 

https://wordpress.stackexchange.com/questions/203962/set-a-default-category-if-the-user-didnt-select-one-before-creating-a-post
https://circlecube.com/says/2013/01/set-default-terms-for-your-custom-taxonomy-default/