The horizontal force on a body moving at constant speed with zero friction is zero, as there is no frictional force to counteract and no acceleration.
The magnitude of the horizontal force acting on a body moving at a constant speed when the coefficient of friction is zero is zero. If a body is moving with constant speed on a horizontal surface, it's not accelerating, which suggests that the net force on the body is zero according to Newton's first law of motion. Since the coefficient of friction is zero, there is no frictional force opposing the motion of the body. Thus, there's no need for a horizontal force to maintain the constant speed of the body. Any existing horizontal force would cause acceleration, which contradicts the premise of constant speed.