How to use the TIMESTAMPADD function

Hi,I need the following:
FieldA = FieldB + FIeldC

where: FieldA = Date field
             FieldB = Date Field
             FieldC = Numeric Field.

I understand that the TIMESTAMPADD function works for this, but I cant get this to work. I am trying to use it in the Defautl field value on FIeldA.

TIMESTAMPADD(SQL_TSI_DAY, "FIELDC", TIMESTAMP 'FIELDB')

How can I do this?

Thanks