I've never created an activation system, but you could always keep the serial numbers in a database of some sort(whether it be online or offline), then query the database for the matching serial number that is inputted into a text box. You may also want to create some type of Key Generator for your use only. Use this to create the keys for your program. Make sure you include a-z,A-Z,1-9, and if you want symbols too.
Hope this helps you get started.
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