If your TM1 instance security is mode 3 (Windows SSO). Arc v4.1.1 introduces support for TM1 security mode 3 (NTLM and Kerberos).

There is no need to do anything on the Arc side.

As a test, if Arc can work or not, check by going into the Browser first and getting into a REST API URL that needs authorization. If that works, then so far, it works with Arc as well.

Given for example,

REST API Port Number: 8782
Security Mode: 3
UseSSL is true

Check first the following if you will be able to automatically be logged in,

https://localhost:8782/api/v1/ActiveUser

Some browsers like Firefox needs extra configuration for it to work.