The ticker symbol for S&P 500 index options is SPX. After entering the ticker select Options followed by the desired contract from the option chains window.
SPX index individual options trade from 09:30 EST to 16:15 EST, but also during extended hours (overnight) from 20:15 EST to 9:15 EST.
Clients may search the IBKR contract database through the Contract Search tool available from the Support area of the IBKR website.