EXEC master.dbo.sp_addlinkedserver
@server     = N'TESTSERVER',
@srvproduct = N'SQLServ', -- it’s not a typo: it can’t be “SQLServer”
@provider   = N'SQLNCLI',
@datasrc    = N'172.x.x.x';
-- Pair local and remote logins
EXEC master.dbo.sp_addlinkedsrvlogin
@rmtsrvname  = N'TESTSERVER',
@useself     = N'False',
@locallogin  = N'Local_user_name',
@rmtuser     = N'Remote_user_name',
@rmtpassword = N'Remote_password';