Sample Azure Key Vault Service Principal Auth Token helper class.

KevVaultUtils.cs

namespace MyDemo.Website.WebApi.Helpers
{
    public static class KevVaultUtils
    {
        // You could also utilise a certificate-based service principal as well
        // Yes, these are hardcoded :)
        private const string kvSP = "xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx";
        private const string kvSPkey = "NOTREALLYTHEPASSWORD";

        public static async Task<string> GetToken(string authority, string resource, string scope)
        {
            var authContext = new AuthenticationContext(authority);
            var clientCred = new ClientCredential(kvSP, kvSPkey);
            var result = await authContext.AcquireTokenAsync(resource, clientCred);

            if (result == null)
                throw new InvalidOperationException("Failed to obtain the JWT token");

            return result.AccessToken;
        }
    }
}