Login to your MessageMedia Hub.
Go to Configuration, then API Settings, then scroll down to Legacy API Credentials and click on Create new key:
In the modal, enter a username for the API (aka a "key") and then a password (aka a "secret"):
Your new API key will then appear in the list where you can edit and/or delete it as required:
source:
https://support.messagemedia.com/hc/en-us/articles/4413619094415-Creating-Legacy-API-credentials