How to populate pick list options of a custom field from external API

How to populate pick list options of a custom field from external API

We need to update the options available to a specific pick list custom field, by getting the current items from an external API.
the API can return any type of data (CSV, JSON, TEXT, e.g.) because we wrote it ourselves.
however, while I'm using the below custom script function, it lets me only add the options that were available during the creation of the pick list field:
  1. $CS.addOptions("WorkOrder_Fields_UDF_CHAR75",["Option 1", "Option 2", "Another Fancy Option!"]); 
I know the custom script is actually JavaScript. I have kinda no problem getting and parsing JSON data during form load, I guess. the issue must be that the $CS.addOptions function does not let me add my own option.

I think this must be possible to be able to achieve something like that. please guide us, as I'm sure this will help a lot of people.

                  New to ADSelfService Plus?