In app designer somewhere when you want to call your function, make sure you have your data ready, and the function is on your path (place it in the same folder as your app).
x = [1:5]
y = [1:5]
[xhat, yhat] = MyOutsideFunction(x,y)
In a seperate .m file, you have your function, but I'll make a dummy function
function [xhat, yhat] = MyOutsideFunction(x,y)
xhat = x * 100;
yhat = y * 100