How do I Generate an ISSN Barcode

I want to generate an ISSN Barcode which is an EAN13 barcode but it has digits 14 & 15 to denote, in my case, the month of issue of a magazine. However, the barcode generator does not let me do this. I also notice that there isn't a light margin indicator [chevron] generated. The specification for an ISSN is shown below and taken from BEST PRACTICE FOR BAR CODING AND ISSUE NUMBERING OF MAGAZINES, PARTWORKS, COLLECTABLES AND PROMOTIONAL VOUCHERS. Updated – June 2014 PRODUCED BY
Professional Publishers Association and Association of Newspaper and Magazine Wholesalers [see here http://www.anmw.co.uk/anmw/documents/PPA_Barcode_Guidelines.pdf]

Magazine issue barcodes are represented in EAN 13 format with a 2-digit add-on code.

• GS1 Prefix (digits 1-3): 977 for magazines with an ISSN

• ISSN Component (digits 4-10): The first 7 digits of the 8 digits ISSN (the last digit, which is a check digit, is deleted as it is redundant when included within the EAN-13 which has its own check digit). The UK Centre at the British Library assigns the ISSN.

• Sequence Variant (digits 11-12): Used on magazines to indicate price changes. For a new magazine start with 00 and move up by 01 for each price change either up or down. Actual prices are not represented anywhere in the bar code. For “one-off” special priced issues an initial sequence variant of 99 should be used and thereafter decreasing the value to 98, 97, 96, etc.

• Check Digit (digit 13): Derived from a calculation, based on the first 12 digits, used to check that the code is correct if manually entered into a system. The Film Master Supplier or your dedicated bar code software would normally calculate the check digit. Please see GS1 UK website for an example of the check digit calculation http://www.gs1uk.org/ - then select ‘Check Digit Calculator’ from the Quick Links menu.

• Add-on (digits 14-15): The 2-digit code used to identify each issue. The section overleaf defines the guidelines for numbering different types and frequencies of magazines. The add-on changes for every issue therefore a new bar code symbol is required for each issue.

• Light Margin Indicator (chevron): This appears in the right-hand margin of the symbol. It ensures there is sufficient margin for the bar code reader to realise it has finished the bar code.