How to create a software activation key

I'm sure there are products to do this - but why not do it yourself?
Initial software activation and trial software becoming full versions etc is not so hard to implement. Your limitations are inherent to the dev environment and platform you use. With one of my MS Access appplications, I've included a boolean field (ActivatedFull) in an Admin table. When my startup routine finds True in this field, it then looks for the activation text file (current directory). If it can't find the file - then I prompt the user to find it for me.

If a file is found or user-provided, then my application will read the text file, determine it's authenticity and release either the extended or trial version of the software, keeping the user informed of what's happening.

Authenticity is the keyword. You will have to come up with your own method (coded character offset replacement, ASCII math calculation with seeded formluas, etc - use the Internet to research known techniques and combine them) of encryption and decryption to build and read the software activation text file.

Then, using your "read text file" code, implement the code into your MS Access's application start up routine.

All the best
Tim