Load the table on sheet B as a mapping table and use the applymap function to add the customer field from the mapping table to the table from sheet A. Something like this:
Map1:
mapping load [Part of address], customer
from ...excel_sheet_b...;
TableA:
LOAD address, applymap('Map1',subfield(address,',',3)) as customer
from ...excel_sheet_a...;
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