Formula or Script to append index of order line items

I have a Orders and Line Items tables that are linked together in the typical parent child relationship where an order has many line items.
My question is how can I append the line item index (+1) to the order number in the line items table?
For example: Order number 20001 has 3 rows of line items in the line items table. I already have a row for each line item in the line items table but I would like a column with 20001-1, 20001-2, 20001-3 ({Order #} + index).
Thanks in advance for any help!

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