How to implement a custom performance function?

I'm trying to implement a custom performance function for an ANN. I've copied the sse and +see objects and renamed them and changed the function name to customsse. These are located in /Program Files/MATLAB/R2018a/toolbox/nnet/nnperformance/. When I train my ANN I get the following error.

Error using network/subsasgn>network_subsasgn (line 551)
"performFcn" cannot be set to non-existing function "custommse"

MATLAB isn't recognizing the new function. I haven't changed anything else about the function yet. I restarted MATLAB and it's reading the directory fine.

If I place the custommse objectives in a personal directory I get the following warning

Warning: Performance function replaced with squared error performance. 

Am I missing something here? Thank you,