You can find API keys on the Dev Tools page.
There are two keys (Public and Secret) and also two modes (Test mode and Live mode).
How they Work
Test mode keys will allow you to interact with test members, and live mode keys allow you to interact with live mode members.
The Live mode API keys will become available once you start paying for Memberstack.
Securing API Keys
I urge you to protect your public and secret API keys. It's important. If your API keys are compromised, unfriendly folks could gain unauthorized access to your data and operations, which could have serious consequences.
Never share your API keys with anyone. Keep copies in a secure space in your applications or on your servers.