Option on a forward price

Suppose the asset underlying a European call is not the share itself but the forward price for buying that share. How would you value such an option?

Use Black's formula: $C=e^{-rT}[F_0N(d_1)-KN(d_2)]$, $d_{1,2}=\tfrac{\ln(F_0/K)\pm\frac12\sigma^2T}{\sigma\sqrt T}$. A forward costs nothing to enter, so under the risk-neutral measure it is a driftless lognormal martingale with the stock's volatility (for deterministic rates). Pricing is Black-Scholes with the forward in place of the spot and zero carry.