popup select quantity

[layman]

Hi! There is a form add to cart code:

Code Select Expand

$href = JRoute::_('index.php?option=com_virtuemart&view=cart&task=add&virtuemart_product_id='.$row['product_id'].'virtuemart_category_id[]=1&quantity[]=1');
adds one number.

How to add a popup with a choice of quantity before adding to cart?

VM 2.0.26d