For ASA version 9.16 and later, Cisco introduced , which eliminates the classic activation key. However, for ASA 9.15 and earlier (still widely deployed), you use the traditional PAK + Activation Key method.
Purchase the required feature (e.g., AnyConnect, Security Plus, Strong Encryption) through an authorized Cisco partner. You will receive a Product Authorization Key (PAK) Evaluation Licenses: You can often request temporary evaluation licenses via the Cisco Software Central Free Features: cisco+asa+license+key+generator+install