Только из командной строки

sqlagent -c [-v]

Arguments-c

Indicates that SQL Server Agent is running from the command prompt and is independent of the Windows NT Service Control Manager. When -c is used, SQL Server Agent cannot be controlled from either the Services application in Control Panel or SQL Server Service Manager.

-v

Indicates that SQL Server Agent runs in verbose mode and writes diagnostic information to the command-prompt window. The diagnostic information is the same as the information written to the SQL Server Agent error log.

Если еще актуально, то данный скрипт создает процедурe, которая будет стартовать при запуске сервера и в свою очередь запускать SQLServerAgent

\nuse master
go
if exists (select * from sysobjects where id = object_id(N'[dbo].[sp_autostart_sqlagent]') and OBJECTPROPERTY(id, N'IsProcedure') = 1)
drop procedure [dbo].[sp_autostart_sqlagent]
GO
SET QUOTED_IDENTIFIER OFF
SET ANSI_NULLS ON
GO
create procedure dbo.sp_autostart_sqlagent
as
declare @retcode int
declare @local int

exec @retcode = sp_OACreate 'SQLDMO.SqlServer', @local OUT
if @retcode<>0
return -1

EXEC @retcode = sp_OAMethod @local, 'Connect', NULL, '(local)'
if @retcode<>0
return -1

declare @js int
EXEC @retcode = sp_OAMethod @local,'JobServer' , @js OUT
return -1

EXEC @retcode = sp_OAMethod @js,'Start'
if @retcode<>0
return -1

return 0
GO
SET QUOTED_IDENTIFIER OFF
SET ANSI_NULLS ON
GO
exec master..sp_procoption 'sp_autostart_sqlagent','startup',true
go