curl --request POST \
  --url https://api.returnshelper.com/uat/user/api/Fba/FbaInstructionReplenish/assignNewFnsku \
  --header 'Content-Type: application/json' \
  --header 'x-rr-apikey: <api-key>' \
  --header 'x-rr-apitoken: <api-key>' \
  --data '
{
  "fbaInstructionId": "3c90c3cc-0d44-4b50-8888-8dd25736052a",
  "assignNewFnskuPayloadList": [
    {
      "oldFnsku": "<string>",
      "newFnsku": "<string>"
    }
  ]
}
'