IAM login : Login failed for user 'test'

I have the same problem as in this post, but in another context.

Some info about our application:

• Built for Windows GUI
• RDMBS: SQL Server 
• Dozens of customers will use our application (installed at their SQL server)
• Users are logging in using ‘Windows authentication’
• We're still on IAM/Indicium 2022.2 (planning to upgrade)
• Indicium is running as a service process, listening on port 5000

What I want to achieve: 

• users log in with their own name, so IAM application roles and rights can be used
• users do not need to be known in SQL security;
  so the SQL connection from the IAM application should be handled by either:
  - a service account;
  - or a SQL account;
  - or an application role.

Question 1:
How can I achieve this for the Windows GUI?

Related to this, I created a user with ‘Authentication’ set to ‘IAM', and changed the password.
I started the application using an INI file containing "Authentication = iam”. When starting, the IAM login shows but after entering username and password, the error “Login failed for user ‘test’ “ shows. In  this post the solution is to add the user to SQL security; however this is an IAM user. How to add an IAM user to SQL security? The actual Windows user starting the application is already known in SQL security and has sufficient rights (because: when starting the application with Windows authentication enabled, all works well).

Question 2:
Why do I get this error?