You can add the split ratios manually to the network's divideParams. Here's a snippet showing the same.
[x,t] = simplefit_dataset;
net = feedforwardnet(10);
net.divideParam.trainRatio = 70/100;
net.divideParam.valRatio = 15/100;
net.divideParam.testRatio = 15/100;
[net,tr] = train(net,x,t);
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