While you receive the superuser (webadmin) login details when your PostgreSQL database node is created, this user isn't recommended to be used directly by your application. For your security, you should create a new user with limited permissions in phpMyAdmin.
Use the Go to pgAdmin button from the dashboard to access the admin panel. Go to the Servers tab and select PostgreSQL. Log in with the credentials you received via email.
Go to the Roles tab and click Create Role.
The login information and privileges are defined in the following step.
You can learn more about user management in phpMyAdmin from the official documentation.