After Sale link to custom sign up page

I need to send customers who purchase certain services to a sign up page to receive a newsletter. As this is only for those who purchase a specific product, I can only make this visible after the payment is processed. How do I set up the attributes to do this? Presently I can only seem to use the download procedure, which is a bit confusing to the customer as there is nothing to download. They are looking for a link to sign up.
Thank you for your help.

You could optionally add a text attribute to that product, and have them fill in their info there before they add-to-cart.
Then you can take that information and use it to add their subscription yourself afterward.

Thank you for your response. Unfortunately, I can't do that. People are eligible to sign up for this information letter only after they purchase the product. Actually it is a subscription to a stock market service. Any other workarounds?
Thank you.

I'm not sure you understood what I said. I'm guessing you've never used product attributes before.
They're purchasing a product. If you add a text-attribute to that product, where the customer fills in that information BEFORE they add THAT product to their cart, then when they complete checkout with THAT item in their cart, the information they entered will be included with the order. And, since the order is never saved until the payment is processed, their information will not be submitted to you until they've purchased the product.

Or, you could hire someone to write custom code to do it all for you. https://www.zen-cart.com/helpwanted

Thank you for your help and your patience. I probably am not giving you all the necessary information. The reason I need to link, after purchase, to a web page is the web page is an opt in sign up for a market letter that is only available to customers who have purchased the service.
We use AWeber as our e-mail delivery vehicle and I need the customer to agree to receive the market letter so his information can be added to the list at AWeber. If I take the information he provides and enter it myself, it is against the rules and I can lose my AWeber account. We do a great deal of e-mail marketing and I really want to play by the rules. That is why I decided not to use the text attribute.
The download addresses my problem and that is what I am doing now. We also sell several downloadable products, so I am uncomfortable making any changes there.
However, it is confusing as we mention the sign up page in the product copy and then the purchaser see a download page after purchase.
I sincerely apologize for not providing you with enough information. I hope this clarifies the issue.
If you have any ideas, they would be greatly appreciated.

Thanks for clarifying.

You'll have to write custom code to send an extra email to those customers, perhaps using an observer class which checks the contents of the order for the specified product and sends the correct email to the customer.

Thanks for your reply. I had a feeling we were going in that direction. I'll get my programmers on it.
I sincerely appreciate your time and help.