Puedes crear un campo calculado:
CREATE TABLE [dbo].[TablaCampoCalculado](
[id] [int] IDENTITY(1,1) NOT NULL,
[tarId] AS ('TAR000000' +CONVERT([varchar],[id])),
[description] [nvarchar](80) NOT NULL,
CONSTRAINT [PK_Nombres] PRIMARY KEY CLUSTERED
(
[id] ASC
)
) ON [PRIMARY]