Replicating ajax add-to-cart call in cart->add_to_cart()

I am trying to replicate the ajax call for adding a product to the cart, and doing it in PHP using the WooCommerce object.
Here is my ajax url that works
https://domain.com/cart/?add-to-cart=857&quantity=1&wc_autoship_frequency=30
Here is my equivalent PHP call
$woocommerce->cart->add_to_cart( 857, 1,'',array("wc_autoship_frequency" => 30) );
This does add the product to the cart, but it ignores the custom value of wc_autoship_frequency.

The wc_autoship_frequency appears to be a variation (value is stored for the product in post_meta table). The product does not have any variation_ids.