For the limited purpose that you have specified you can use:
Thrust Horsepower = (Thrust in lb x TAS in Kts) / 325.658
This is based on:
Thrust Power = Thrust x TAS
1 horsepower = 33000 ft lb/min
1 Kt = 6080 ft/hour
But it cannot be used to estimate BHP of a piston engine or SHP of a turbojet. As others have stated previously this also depends on propeller efficiency which is not constant.