As per the mentioned scenario, you would like to use API keys. For that, I would recommend to use Integration key as API keys have now been deprecated. You can generate Integration Key by going to Account->Adobe DC esign API and click Integration key.
One can use Oauth and the resulting access code instead of an api key.
To get the integration key option though (which is a fixed access code if you like) contact support.
To contact support login (ideally as the admin of the esign account) your account, click the question mark
click the get started link under contact support
then select one of the 4 options to contact support.
Integration key is superior to using API Key in terms of security and flexibility. First, you can easily generate a new integration key and revoke the previous one, if say the previous one has been exposed to unauthorized personnel. Secondly, you can assign different permission levels to each integration key instead of allowing access to all permissions like the API key. So for different access level, you can fine tune your API calls to ensure proper access.