Unit testing MVC Controller making calls to a static library calling external API

Good day,

I am confused about unit testing the following:

1. MVC Controller:

[HttpGet]
public async Task<PartialViewResult> DoExternalCallsAsync()
{
    var model = new MyModel();

    await MyStaticLibrary.DoExternalWorkAsync(Server.MapPath("~\\") + "WorkSource.txt", model);

    return PartialView("_MyResults", model);
}

2. Static library:

public static async Task DoExternalWorkAsync(string sourcePath, MyModel model)
        {
            var externalCalls =
                System.IO.File.ReadAllLines(sourcePath)
                .Where(line => (!string.IsNullOrEmpty(line) && line.First() != '#'))
                .Select(p => DoExternalCall(p, model));

            await Task.WhenAll(externalCalls);


        }



 private static async Task DoExternalCall(string urlPath, MyModel model)
            {
                var result = await GetExternalApiResultAysnc(urlPath);

                // some code here...



                return;
            }

Basically, all that the controller does is call an external API, which does some work and returns a result or throws an error.

There are no interfaces or abstract classes with the external Api.

How do I go about unit testing this? N. B. I am not at liberty to change the design of the external Api.

Thanks,