I was also searching for this info, but I was not able to find the answer how this is done. Same mechanism is used for an example for the Spacecoins or the newest one Hosky. I do not think you need smart contract for this but I am not sure.
I think this is simply done by the bash script running on some server with synced wallet. You can basically monitor your wallet, once you will receive the ADA you can filter the wallet ID using cli and construct outgoing transaction. And that's it.