const string KEY_CLAIM = "http://schemas.microsoft.com/2017/07/functions/claims/keyid";

public static async Task<IActionResult> Run(HttpRequest req, ILogger log)

{

    var claim = req.HttpContext.User.Claims.FirstOrDefault(c => c.Type == KEY_CLAIM);

    if (claim == null) 

    {

        log.LogError("Something went SUPER wrong");

        throw new UnauthorizedAccessException();

    }

    else

    {

        log.LogInformation( "Processing call from {callSource}", claim.Value);

    }