I am not sure what you mean with "formula" here. To me it seems as task of joining the tables by matching the zip codes.
The Update function useful for such a task.
Manually:
Activate the first table and invoke Update from the Tables menu. Slect the second Table in the list and check "Match Columns". Then select Zip Code and ZIPN as matching columns and hit OK. If second table contains other columns that you don't want to be added to the 1st table you can select only the ones yo need.
By script:
dt=datatable("1st Table");
dt << Update(
With( Data Table( "2nd Table" ) ),
Match Columns(:Name("Zip Code") = :ZIPN)
);