The "target" interest rate is whatever it takes to sell them.
who will buy those bonds from the Fed?
The Fed can hold the bonds until maturity.
But then, on that day, the Treasury will have to redeem the bonds at face value. Whereupon they will have to either get some money from the tax payers or borrow some money from folks interested in buying new bonds or sell new bonds to the Fed if all of the other bond buyers are demanding an interest rate that is outside of where the Treasury needs it to be.