The automatic creation of barcodes in MET/TEAM is governed by three system defaults.
Barcode Length is the number of total digits in the barcode. This includes the Prefix, serialized number, and Suffix.
Barcode Prefix is the first letters of the barcode which come before the serialized number. In both the default setting and your example, this is PRE.
Barcode Suffix is additional characters added to the end of the barcode, which it doesn't appear you are wanting to use, so just leave it inactive.
The current counter can be changed with an SQL script:
SET cCurrentCount = '12345'
WHERE cTableName = 'BARCODECOUNTER'
This would make the next barcode generated with my settings "PRE00012346."