Automatically assign taxonomy term if custom meta value exists

I currently have a custom meta field for adding a video url to the post. I’d like for the existing taxonomy term “video” to be automatically assigned to the post upon saving if the meta field has any value.

You should hook onto the save_post action.

add_action( 'save_post', 'add_video_taxonomy' );

function add_video_taxonomy( $post_id ) {

    // you should check if the current user can do this, probably against publish_posts
    // you should also have a nonce check here as well

    if( get_post_meta( $post_id, 'video_url', true ) ) {
        wp_set_post_terms( $post_id, 'video', 'your_custom_taxonomy_name', true );
    }

}