Get custom taxonomy term url on archive page

If you are in term archive page, the taxonomy and term is actually set. You could hook wp_head to add the open graph in your functions.php:

 add_action( 'wp_head', 'wpse_wp_head' );
 function wpse_wp_head () {

      //First, we check if we are in our custom taxonomy
      if( is_tax('my-custom-taxonomy') ) {

           $taxonomy_slug = get_query_var( 'taxonomy' );
           $term_slug = get_query_var( 'term' );
           $term_url = get_term_link( $term_slug, $taxonomy_slug );

           //Add Open Graph property
           echo '<meta property="og:url" content="'. esc_url( $term_url ) .'">';

           //If you need the full object of current term
           //$term = get_term_by( 'slug', $term_slug, $taxonomy_slug );
           //Now you can get all the term data. For example:
           //$term->name, $term->term_id, $term->slug,.....

      }

 }