Custom-mapping the barcode field when importing MARC records to LT

Since I am new to this process and weak in the area of tech services(my background is in public services and library management), I have a question regarding custom-mapping of barcodes when importing MARC records to LT. Specifically, how is it done?
My books come processed from the vendor with a code 39 barcode symbology. How do I custom-map these barcodes for circulation purposes in TinyCat?
Sorry for the simplistic question. I am starting-up a small high school library and will be using LT and TinyCat as out ILC/OPAC.
Thanks in advance for any tips/advice you have.

Best,
Tony Powers
DePaul College Prep

Please see my reply here: https://www.librarything.com/topic/334116. Thanks!