If you manage multiple accounts, have multiple linked accounts, or use account partitions, you will be prompted to select one or more accounts when you submit orders in TWS:
Classic TWS - In the upper part of the quote monitor you see a Select Account drop-down menu that allows you to select the default account where the orders you create on the page are allocated to. Then, click the Ask or Bid in a market data line to create an order line directly below the selected ticker. Click the Ask to create a Buy order. Click the Bid to create a Sell order. On the Order line, modify the order parameters including Account, if needed. To transmit your order, click "Transmit" (or "T" if you have reduced the size of the Transmit field).
Mosaic - Within the Order Entry panel you see a green Account drop-down menu just below the order quantity row, that allows you to select the account you want to allocate the order to.
More information on how to create an order for multiple clients in TWS can be found in our Users' Guide.