Simulating nested params in tests

I need simulate sending params in test, same as these inputs would send:

<input id="quantity" name="quantities[9671]" type="number" value="1" />
<input id="quantity" name="quantities[9822]" type="number" value="1" />

Test looks like this: (using Test Unit, the default testing framework in Rails):

test "placing item into basket"
  cart = Cart.new(session)

  post :create,
    product_variant_ids: [@product.variants.first.id],
    "quantities[#{@product.variants.first.id}]" => 1 # <<<------ not working 

  assert_equal 1, cart.items.count
end