Strategy Builder has several pre-defined option combinations available from the Strategy dropdown menu to the lower right of the screen. Reveal the list by clicking on the dropdown menu. Select Straddle from the list, and notice the icon shown in the lower part of the display instructing you to click on a leg to display the straddle.
When the cursor is placed above the bid price of a call, the software automatically highlights the element for the rest of the combination. For a short straddle, TWS knows that selling a call requires the sale of a put at the same strike and expiration. Hence, both legs display in red.
Locate the strike you want to build your short straddle for and click on the bid price of the call. Note that a straddle combines the same strike and same expiration call and put.
Use the Order Entry panel at the bottom of the page to create the order. Define the quantity of straddle combinations to trade. Click the Submit button to generate an Order Confirmation. When you have reviewed the details, click Transmit.
Source: (IBKR, 2023, PRACTICAL USAGE – NEUTRAL MARKET – SHORT STRADDLE, https://ibkrcampus.com/trading-lessons/practical-usage-neutral-market-short-straddle/ )