Get Custom Field API Name

Get Custom Field API Name

I am trying to make a custom function to connect a Zoho Survey score into a custom field we have in a task layout for the QA Score. I need to input the custom field API name on the script, but I don't know how to get it. I've tried using the following script to get it but when I execute it the specific custom field I need is not showing up in the execution response even though the other ones are there. 

var = zoho.projects.getPortals("zprojects");
portalId = var.getJson("portals").getJson("id");
projectId = "";
taskId = "";
taskFields = invokeurl
[
url :"https://projectsapi.zoho.com/restapi/portal/" + portalId + "/projects/" + projectId + "/tasks/" + taskId + "/"
type :GET
connection:"zprojects"
];
info taskFields;
return "success";