Hi,
You shouldn't have to specify a page as HikaShop already sends to PayPal the URL of the page to redirect back the customer.
Also, it would be better to use the normal thank you page of HikaShop in your PayPal merchant account if you want GA to work.
If you use your own thank you page, then the GA code from HikaShop won't be added to the page.
Then, you can enter your own thank you page in the "return URL" setting of the payment method. That way, after displaying the HikaShop thank you page (with the GA code on it), HikaShop will redirect to your return URL.
The normal thank you page of HikaShop is: yourwebsite.com/index.php?option=com_hikashop&ctrl=checkout&task=after_end