msi-func.csx

using System.Net;
using System.Net.Http;
using System.Net.Http.Headers;
using Microsoft.Azure.Services.AppAuthentication;

public static async Task<HttpResponseMessage> Run(HttpRequestMessage req, TraceWriter log)
{
    var target = "<target app client ID>";
    var azureServiceTokenProvider = new AzureServiceTokenProvider();
    string accessToken = await azureServiceTokenProvider.GetAccessTokenAsync(target);

    log.Info($"Got token for {target}: {accessToken}");

    var wc = new System.Net.Http.HttpClient();
    wc.DefaultRequestHeaders.Authorization = new AuthenticationHeaderValue("Bearer", accessToken);
    var result = await wc.GetAsync("https://<app endpoint>/api/who");

    var data = await result.Content.ReadAsStringAsync();
    log.Info($"got {data}");

    return req.CreateResponse(HttpStatusCode.OK, data);
}