Once purchased, the system should send you a couple of letters from the store. There you will see the bill report and also the license key.
If you didn't get a letter, please check your spam box. Also, you can find your license key on your account page at https://fulltextsearch.org/my-account/.
If you still have issues with it, please put a message in the Live Chat on the https://fulltextsearch.org main page. Thanks!