The instructions you linked will only work with a Common Thank-you Page URL, not with product-specific Redirection URL. If your digital product is a download, you cannot combine that with a product-specific Redirection anyway; enabling one disables the other and vice-versa.
However, it's not necessary to do any of that; you can simply customize your E-junkie-generated thank-you pages to add your tracking code. To add the same code for all your products, just go to Seller Admin > Account Preferences > Common Thank-you Page HTML and paste your tracking code in there; however, if you need to add the code for only a specific product, put it in the Message (HTML) field of that product's settings.
If your product is not a download, buyers may not ever choose to view any thank-you page, so in that case you may want to use our Cart instead of Buy Now buttons, so you can use the technique described on our Tracking help page here: