Here is a little project I created to populate a single selection (radio in this case) option in vRo workflow from a Device42 Customer API call. The idea being that you could query out via rest, get a list of predefined customers and use that as the selections.
The challenge is that the input form external field needs an action and not a workflow. I basically reverse engineered the "Invoke Dynamic REST Operation" workflow's scripted task.
While the example here uses Device42 the concept and code would be relatively similar for any type of REST API with a JSON content returned.
This took some work. Here are the fruits of the labor.
Create getDev42Customers vRo Action using script at the bottom with the required inputs shown below.
Configure the input form with External value and the getDev42Customers with its input
The action setup will result in the list array being populated by the Customer Names
And here is embedded code.