Are you asking for creating third column in the table ?
or If you want only select query to view the result, you can use this following query.
WITH CTE as
(
select 'TS-SIV-VS-' as Prefix, 1 as srno
union all
select 'TS-SIV-VS-' , 4
union all
select 'TS-SIV-VS-' , 1
)
select Prefix,Srno,prefix+cast(srno as nvarchar(2)) Invoice from cte