SQL Server 2012 :: Get Next ID With A MERGE?

I have a table with a field called SeqId which is not an identity nor a sequence but a kind of autonumber field (max(SeqId) + 1). Now I have to do a MERGE between 2 tables where the one with SeqId is the target.

How can I get the next SeqId for every row added? I tried this:

MERGE dbo.CRM_MNP_ORIGINAL_NRN AS T
USING dbo.seriesnacionales AS S
ON (T.RANGE1 = S.RANGOINI )
WHEN NOT MATCHED THEN
INSERT (SeqId, Range1, Range2, OPERATORCODE, NRN, StartDate, CreateDate)
VALUES((SELECT dbo.FN_GetNextSeqId4CRM_MNP_ORIGINAL_NRN()), S.RangoIni, S.RangoFin, S.IdOperador,
'TEST_M', Convert(DATETIME, FECASIGNA , 103), SYSDATETIME())
WHEN MATCHED THEN
UPDATE SET T.Range1 = S.RangoIni, T.Range2 = S.RangoFin, T.OPERATORCODE = S.IdOperador
OUTPUT $action, Inserted.*, Deleted.*;
where the function just return Max(SeqId) + 1

but I always get the same value for that field.