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Looks like upon receiving your phone, you register your Note 20 IMEI in the Samsung Shop app and upload your Best Buy receipt info by a certain date to get the $150 Samsung credit (for the Note 20 Ultra, $100 for the Note 20). The process is outlined in the terms and conditions of the preorder ad listed on the Bestbuy.com homepage.
I asked the Samsung rep today and was told it has to be order off Samsung.com to get the credit.