how to use a trained neural network as a fitness function in GA?

We have done an experiment regarding weight of a system. We have to optimise (find the minimum) the weight of the system involved. The experiment involves three parameters. The size of input is 3x79 and the size of output is 1x79. Using these data we have created a feedforward backpropagation with 10 neurons(1 hidden layer). As our problem doesnt have an objective function we need this trained network to act as a fitness function in Genetic Algorithm for optimisation within the specified range of our input.can any one please explain how to do this?