How to modify a built-in in function that is a parameter of another like functionA('functionB') AKA use a custom training function for a neural network

The network creation

net = feedforwardnet([50],'traingd');

called the function traingd.m, there is a set of these functions.

Now I want a custom trainFunction but they does not allow me to create a function with another name

net = feedforwardnet([50],'trainscgXX');

Error using feedforwardnet (line 68)

FCN does not return an info object.

There is a suggestion like

% To change a neural network's training algorithm set the net.<a href="matlab:doc nnproperty.net_trainFcn">trainFcn</a>
% property to the name of the corresponding function.  For example, to use
% the scaled conjugate gradient backprop training algorithm:
%
%   net.<a href="matlab:doc nnproperty.net_trainFcn">trainFcn</a> = '<a href="matlab:doc trainscg">trainscg</a>';

I can't understand how to make it, it seems that there is a limited number of function that it can accept as input, and I don't know how to add also mine.

Thanks

p.s. I know that I can modify one of the one that works, but that is not the choice I want to make