If you reuse this article in your ticket, ask the customer why it is important for them to use additional admin account for API instead of built-in admin user. Forward the customer's response to @ipostnikov
Executing REST API request to Plesk server under additional administrator account fails:
"message": "Can not find client by login 'johndoe'"
Plesk REST API limitation: requests cannot be executed under Plesk additional administrator accounts.
The item for improvement with ID EXTREST-87 was created to implement this functionality.
Until the functionality is implemented, execute the API request using built-in "admin" Plesk account instead, for example:
# curl -k -u admin:password -X POST -d @domain.json "https://203.0.113.2:8443/api/v2/domains"