How to access manage discount list using sales order extension

Summary: We have a requirement to add discount line in a sales order based on ship-from and ship-to condition. To achieve this we have defined zero price of item in the price list and defined a discount amount in discount list based on ship-from and ship-to. Now to add the line we are going with extension but I am not sure how we can access manage discount list to put the condition of ship-from and ship-to. Kindly help me with some sample which I can refer.

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Version (include the version you are using, if applicable): 23C