To Add New Database into Always-on Group in Existed AGgroup using T-SQL command

1. To connect Primary Replica Instance and copy the below script and paste into the Query window. To change the below are the changes as per your requirement.

PrimaryDBinstancename,
SQLPort
AGname
SecondaryDBinstancename.

--- Example: T-SQL code

--- YOU MUST EXECUTE THE FOLLOWING SCRIPT IN SQLCMD MODE. in the query e
:Connect PrimaryDBinstance,1433
USE [master]
GO
ALTER AVAILABILITY GROUP [AGName]
MODIFY REPLICA ON N'SecondaryDBInstance' WITH (SEEDING_MODE = AUTOMATIC)
GO
USE [master]
GO
ALTER AVAILABILITY GROUP [AGName]
ADD DATABASE [Test];
GO
:Connect SecondaryDBInstance
ALTER AVAILABILITY GROUP [AGName] GRANT CREATE ANY DATABASE;
GO
GO