Internal Import Adding A New Field To Mapping

I am trying to import a series of test cases to Zephyr Squad using the Internal Upload. 

As part of the documentation for Internal Upload they show "Epic Link" as one of their fields to be uploaded, but in the JSON file example, they have it mapped as 

"customfield_10014": "Epic link" 

When I look at the "sample_mapping" png it shows the Epic Link as available to be mapped to a value in the file - 

When I look at my mapping screen, I do not see that available for me to select. 

How do I add either Jira fields or custom fields to the Mapping screen for the internal Import of test cases?