I'd recommend that you look at some of the product listing pages and product_info page to get a sense of the sort of data that needs to be passed in the URL. In particular note that the page is generally product_info rather than shopping cart. Zen Cart will determine for itself (based on an Admin switch) whether it is appropriate to invoke the shopping cart after adding items to it, or whether it should go onto the page that you specify.
However, DrByte's point is that even if you get all the parameters passed in the URL correct, that won't be sufficient. You will still need to pass the cart_quantity and products_id via the $_POST variable (please note that "s" in products_id).
Not being a Flash expert, I can't tell you how to do this, or even whether it can be done (though I would be very surprised if it couldn't).
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