You can create a bracket order (OCA) for an existing position by adding the OCA Group column to your TWS. Right click on the tab header and choose Layout. Make sure you are on the Order Columns tab and find the Order Attributes section on the right under Available Columns. Click the green plus sign (+) to expand the section.

Click OCA Group and click the Add button in the middle. You should now see the OCA group column on the left side under Shown Columns. Once finished, click the Apply and OK buttons at the bottom of the window. Once the columns are added, you can create an OCA order by creating the orders you wish to link and entering an identical identifier in the OCA group column.
For example if you wish to link a profit taking sell limit order and a stop loss sell stop order, you would create the two orders and entering a matching identifier (ex. trade123) in the OCA group field.
Once finished, transmit both orders.