how to use a series (which do not have update field) in another series's server expression.

suppose we are creating a new series  ''series_C'' and formula used should be simply  "series_A" plus "series_B".

and we are supposed to use server expression for "series_C".

but the problem is series_B do not have update field (note: it has both client & server expression but no update field in Data properties).
so how to refer series_B in server expression of series_C.

I mean what to write in server expression like branch_data.%%%  .

Please suggest.