First, you need to convert your IBKR account to Portfolio Margin. You need to put much more than the minimum $100K equity requirement.
On IBKR mobile app's watchlist look for SPX. Click Options. Scroll to desired date you wish for the tenor of the loan (the longest for now being Dec 14, 2023). Turn on the Strategy Builder at the bottom so you can see what you choose. Scroll on the option strike price to determine the desired width of the box (amount of the loan) e.g. 4000 - 5000 (1000 distance x 100 = $100,000.- loan to be paid back at the expiration date)
On 4000 strike price
1. Click Ask on the left side : Buy 1 4000 Call
2. Click Bid on the right side : Sell 1 4000 Put
On 5000 strike price
3. Click Ask on the right side : Buy 1 5000 Put
4. Click Bid on the left side : Sell 1 5000 Call
Add these transactions to your favorite Watchlist. The SPX 4000 - 5000 Box will be shown with the corresponding Bid and Ask and including how many contracts on offer (1 contract is $100k). Try to sell the contract (or as many contracts desired) on the Box as high as possible to reduce the interest that you will be paying (ie. should be higher than the Bid and lower than the Ask). Remember you will repay $100k x the numbers of contracts, by the end of the term. Please also remember how much equity value you have when trading the amount of the loans/contracts and also that you will need to repay those loan at the end of the term.
Please post what you have accomplished. You may want to try on a simulated account.
You should ditch your Schwab account.