There are 2 different kinds of API key's within your Myphoner Account, the personal one which can be used to only affect your account or the account-wide API key.
Account API Key
This API key is used on an account level and affects all users, only admins have access to the API key.
To find the account API key go to Manage, select Integrations & API.
Please note that the account API key will give you access to all of Myphoner. Don't give it to anybody that you don't trust!
Personal API Key
To find your personal API key simply go to My Preferences, select Credentials and your API key will be there.
Please note that your personal API key will give you the same access rights as your user has.