Hello,
The volume discount API example is available under ~\YourProjectFolder\CMSApiExamples\Code\Ecommerce,  #region "API examples - Volume discount" on line 2978:
   private bool CreateVolumeDiscount()
    {
        // Get the data
        var product = SKUInfoProvider.GetSKUs()
                           .WhereStartsWith("SKUName", "MyNew")
                           .FirstOrDefault();
        if (product != null)
        {
            // Create new volume discount object
            VolumeDiscountInfo newDiscount = new VolumeDiscountInfo();
            // Set the properties
            newDiscount.VolumeDiscountMinCount = 100;
            newDiscount.VolumeDiscountValue = 20;
            newDiscount.VolumeDiscountSKUID = product.SKUID;
            newDiscount.VolumeDiscountIsFlatValue = false;
            // Create the volume discount
            VolumeDiscountInfoProvider.SetVolumeDiscountInfo(newDiscount);
            return true;
        }
Please let me know if you have any additional questions about this.
Best Regards,
Sandro