When WordPress instance is deployed via WordPress toolkit, database name and database username are set automatically.
How to define such settings manually?
2 options are available:
Using WordPress Toolkit.
To install WordPress with the custom settings, go to Domains > example.com > WordPress > Install > Install (Custom):
From the application catalog.
Also, it is possible to deploy WordPress with custom settings from the application catalog in Domains > example.com > Applications > Featured applications > WordPress > click drop-down on Install > Install (Custom):