You’re trying to pretend that you own the underlying.
To do so, you take the long position in a futures contract, and buy a bond that matures on the day the futures expires; the par value of the bond will be the futures price, so yo have enough to pay for delivery. It’s a risk-free bond, so you haven’t introduced any additional risk.
Note: this doesn’t work if the underlying has cash flows between the date you enter into the futures and the expiration date.