How to track Adwords user and pass a variable into the downloaded software?

What I want is to calculate the ROI of a campaign and this is my scenario. A user comes from Google Adwords into my webpage and then downloads a software. The software is coded in C#. I know that there is a conversion option from Google that registers the action when the download button is clicked. But this is not the end because the ROI is not calculated when the software is downloade, but from his spending inside the software.

However, my question is more technical than that. What I want to achieve is to "marry" the adwords visitor with his downloadable software. Meaning, to pass a unique code (or anything that works maybe a cookie) from the webpage to the software.

Some possible answers that lack extended knowledge from my side are:

1. Create a cookie with a unique id when a user enters the webpage from google adwords, and then somehow the software will read it. As far as I know a software can not read a cookie (I may be mistaken) but what if on software's first launch opens a webpage on his browser?
2. Make use of IP address. So, if a user comes from google adwords, get his IP, save it into a database and the same happens when he opens the software. Theoretically you have a match.

Did you encounter such things? Do you know any way that actually works?

 

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but it solves other problems