Measure for Multitarget Neural Network Regressor

Hi all!
I am stuck as I need to find a measure for MultitargetNeuralNetworkRegressor to tune my model. With NeuralNetworkRegressor I use rmse but it seems like it does not work for this one and I can’t find the corresponding measure.

I use MultitargetNeuralNetworkRegressor because I need a vector in the output.

Thanks for the help and do not hesitate to ask me to clarify if needed.