string authority = "https://login.microsoftonline.com/{0}";
string graphResourceId = "https://graph.microsoft.com";
string tenantId = "tenant id";
string clientId = "client Id";
string secret = "sercet key";
authority = String.Format(authority, tenantId);
AuthenticationContext authContext = new AuthenticationContext(authority);
var accessToken = authContext.AcquireTokenAsync(graphResourceId, new ClientCredential(clientId, secret)).Result.AccessToken;
var graphserviceClient = new GraphServiceClient(
                new DelegateAuthenticationProvider(
                    requestMessage =>
                    {
                        requestMessage.Headers.Authorization = new AuthenticationHeaderValue("bearer", accessToken);

                        return Task.FromResult(0);
                    }));
var dic = new Dictionary<string, object> {{"@odata.type", "microsoft.graph.invitedUserMessageInfo"}};

Invitation invitation = new Invitation
            {
                InvitedUserEmailAddress = "email address",
                InvitedUserMessageInfo = new InvitedUserMessageInfo{AdditionalData = dic },
                InvitedUserDisplayName = "xxx",
                SendInvitationMessage = false,
                InviteRedirectUrl = "xxxxx"

            };
 var result = graphserviceClient.Invitations.Request().AddAsync(invitation).Result;