here is what I did to retrieve the custom data 
1- create a function in stitch functions.
the function you create looks like this:
exports = function(arg){
return context.user.custom_data;
};
2- you give it a name example: myFunction
3.you call the function inside your code like so:
const client = Stitch.defaultAppClient;
client.callFunction(‘myFunction’).then(result=>result).catch(err=>err)
I hope this answers your questions, good luck 