It depends which model hub you have. The ADT model has its own key Fob, but it only works with that model.
Other than that, either the Lowe’s Iris that @Automated_House mentioned or one of the Aeotec Keyfob models are pretty popular. The Lowe’s is zigbee and the Aeotec are zwave. ( note that only some lowe’s Models work, so check the forum for the specific model numbers.) There are also some buttons that work cloud to cloud.
You can see all of these and more in the buttons FAQ:
https://www.amazon.com/Aeon-Labs-ZW088-Aeotec-Z-Wave/dp/B01M1DY8AK
https://www.lowes.com/pd/Iris-Iris-White-Wireless-Home-Automation-Fob/999925322
