This is a very broad question with a lot of different solutions, which might the reason that no one has responded yet. I will give your question a shot below, but without knowing a lot of information about if the optin page was created using Infusionsoft, a different third party solution, or directly in WordPress, please keep in mind that I might not be giving you the best or most applicable answer.
If you’ve created the optin page directly in WordPress my suggestion is to use Google Analytics and set an evetn for the button to the sales page. It may seem daunting at first, but Google Analytics is the industry standard for this type of tracking. Here is a great article that should help you with this exact situation.
You also may want to look at this plugin. I have never used it, but it seems to simplify part of the event tracking process in Google Analytics.
Please feel free to contact me if you have any further questions about this.