You can try the following APIs:
Create an API Key — MongoDB Ops Manager 6.0 to create an organization API key (note, that the user making the call must have enough permissions for this operation)
or Create and Assign One Organization API Key to One Project — MongoDB Ops Manager 6.0 to restrict the org API key to one specific project only.
You may need to use the other APIs (to create organization/project) the same way.
To answer you question about full automation - in your CI/CD you can use the global API key that is created once the OpsManager is created in Kubernetes. During OpsManager resource creation the Operator initializes the first user there and saves the API keys to the secret with format “<om_resource_name>-admin-key”. After reading you will be able to access OM as a global admin