Your customer number can be found on your mailing label, as indicated in these samples:
#BXNMTHDJDJ ************** AUTO**5-DIGIT 01234
#123456781# 12345678 PS2 Q 10/6/22 1501
JOHN DOE
JOHN DOE COMPANY
123 ELM ST
SPRINGFIELD, YZ, 01234-5678
PS2 12345778 Q
JOHN DOE
JOHN DOE COMPANY
113 ELM STREET
A-1234 DORNBIRN
AUSTRIA