Your JSON body content doesn't match the required structure. The
orderLegCollection
needs to be a collection of objects, but you've only passed a single object.
Try:
postRequest.AddJsonBody(New With {Key .session = "AM", Key .duration = "FILL_OR_KILL", Key .orderType = "MARKET", Key .orderStrategyType = "Single", .orderLegCollection = New Object() { New With {Key .instruction = "BUY", Key .quantity = 1, .instrument = New With {Key .symbol = "MSFT", Key .assetType = "EQUITY"}}}})