var graphClient = this.graphService.GraphClient;
            var reqUrl = graphClient.Me.Request().RequestUrl;
            reqUrl += "/findMeetingTimes";
            var headers = new Dictionary<string, string>
            {
                { "Prefer", $"outlook.timezone = "{parameter.PreferredTimeZone}"" },
            };

            var noOfMeetingSlots = parameter.MaxCandidates ?? 5;

            parameter.MaxCandidates = 25;

            var hrm = this.graphService.CreateHttpPostRequest<MeetingSuggestionParameter>(
                    parameter,
                    new Uri(reqUrl),
                    headers);

            await graphClient.AuthenticationProvider.AuthenticateRequestAsync(hrm).ConfigureAwait(false);

            MeetingSuggestionResult result = null;

            try
            {
                using (HttpResponseMessage response = await graphClient.HttpProvider.SendAsync(hrm).ConfigureAwait(false))
                {`

But this does work using the Client SDK. The Following snippet Works using client sdk.

`var re =  await graphClient.Me.FindMeetingTimes(Attendees, null, null, new Duration(parameter.MeetingDuration), parameter.MaxCandidates, parameter.IsOrganizerOptional, parameter.ReturnSuggestionReasons, parameter.MinimumAttendeePercentage).Request().PostAsync().ConfigureAwait(false);