To create GTIN's for use on coupons the digits '99' are used instead of '94' at the start of the GS1 prefix. Where the coupon relates to an existing product the GTIN of the product is used as the basis of the coupon number.
This means when you look at your numbers, swap the 4 for a 9, eg: 94XXXXXXXXXXc to 99XXXXXXXXXXc.
Where the coupon does not relate to a single product, you would create a new product with full description in ether Product Vault, My Products or your own spreadsheet / database of GTIN's as a '94' number, then manually change out the 4 for a 9.
Don’t forget to change the check digit on the end to match the new GTIN number after swapping the 4 for a 9. This can be done with our check digit calculator here. If you are unsure what the check digit is, please read our brief description here