Woop! solution found: please note I’m using ACF plugin for storing metadata. https://www.advancedcustomfields.com/
function create_category( $term, $req, $creation ) {
$params = $req->get_params(); // holds the request body
update_field('key', 'value', 'product_cat_'.$term->term_id);
}
add_action( 'woocommerce_rest_insert_product_cat', 'create_category', 10, 3);