Syntax clumsy but at least the formula is built-in.
a = Quantity[4*10^14, "Hz"];
b = Quantity[273, "Kelvin"];
FormulaData[{"PlanckRadiationLaw", "Frequency"}, {QuantityVariable["\[Nu]","Frequency"] -> a, QuantityVariable["T","Temperature"] -> b}]
I(\[Nu])==2.728045498776555*10^-37 kg/(s)^2