To force the product page on Bigcommerce to load a specific variation product you can append the sku value as a query string. You can build a feed rule to do this for you in the feed using the concatenation function:

Here's what the rule will look like

SET [F].[link] (or whatever the feed product URL field is called)

TO [P].[Product URL] + ?sku= + [P].[sku]

WHERE [P].[Product URL] is not empty

 Note, ?sku= is static text, while the other fields are dynamic from your product catalog

Here's what the final value will look like

https://www.shopname.com/product-handle/?sku=123