License Settings is located under My Calendar > My Calendar Pro > License.

License field showing a saved and active license key for a lifetime license.

You can activate your license here.

If you’re using a license on multiple sites, you may find it easier to define your license key using the constant MCS_LICENSE_KEY. If this constant is defined, the value will be used to activate and verify licenses for the domain.

Note: Be sure to store your license key somewhere secure! You can set it up in your code as a secret in Github or similar options in other repositories, so you don’t expose it publicly.