netc = closeloop(net);

[xc,xic,aic,tc] = preparets(netc,X,{},T);

yc = netc(xc,xic,aic);

tcs = size(tc,2);

closedLoopPerformance = perform(net,tc,yc)

where 'X' and 'T' are input and target vectors

'net' is narx network that is already created by you