How do I protect my application?

welcome
I am currently working on an application and I want to make it paid and I am thinking of protecting it from illegal copies by preventing the application from working until after the application extracts the serial number from the device and presents it to the buyer (the owner of the device).. The buyer sends the serial number of his device to me.. so I By sending a code based on the serial number of his device.. and when he enters it in a field designated for that and presses the confirm button.. the application works only if the code he put is correct.. how can this be done using appinventor?
Gratefully

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Alternatively use this extension -


You can check if app is installed from playstore etc. if somebody has downloaded the apk it will not work
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Many thanx, I'll try it
If it works it will be very helpful