There are two places you can enter custom text for the E-junkie thank-you page -- as a general preference applicable for any product, and on a per-product basis (on the last product config screen before you get the codes). Also make sure that you have marked the Redirection checkbox for each product you want those going to your thank-you page instead of ours (I think this is only necessary if you want to provide your own thank-you page that is a different page for every product?).
BTW, also note that there is no way to "force" buyers to visit your thank-you page. After completing payment, the payment processor typically pauses to inform the buyer that their payment has been successful, waiting for the buyer to notice that message and then take some action to acknowledge it and proceed. Although the processor's payment-confirmation page offers a continue link that would go to your thank-you page, the buyer doesn't need to click that link and may instead choose to visit some other site or just close the window.