https://docs.microsoft.com/en-us/sql/database-engine/database-mirroring/database-mirroring-establish-session-windows-authentication

Prepare a Mirror Database for Mirroring (SQL Server)

https://docs.microsoft.com/en-us/sql/database-engine/database-mirroring/prepare-a-mirror-database-for-mirroring-sql-server

SELECT role_desc, state_desc
FROM sys.database_mirroring_endpoints;

Database Mirroring - Allow Network Access - Windows Authentication

https://docs.microsoft.com/en-us/sql/database-engine/database-mirroring/database-mirroring-allow-network-access-windows-authentication

USE master; 
GO 
CREATE LOGIN [Adomain\Otheruser] FROM WINDOWS; 
GO 

GRANT CONNECT on ENDPOINT::Mirroring_Endpoint TO [Adomain\Otheruser]; 
GO