For any SMS Service that is created, you can create multiple API keys for that service for multiple applications that you would like to use it in.
You can manage or create a new API key by navigating to the SMS Service in question and under the API Key menu, you will be able to view the available keys.
In active API keys will automatically be decommissioned after 6 months of inactivity for security reasons.
SMS API Keys section with the "Generate API Key" button visible
To utilise the Atomic SMS API, you will need to use your custom domain to call the various SMS API endpoints. You can get your custom URL on the same page as where you can set your API key, just scroll further down under Service Details > SMS API URL.
SMS API URL field under Service Details with a redacted custom domain and copy button