CouponModel.cs

using System;
using System.Net.Http;
using System.Threading.Tasks;
using Microsoft.AspNetCore.Mvc;
using Microsoft.AspNetCore.Mvc.RazorPages;
using Newtonsoft.Json;
using Serilog;

namespace WebPortal.Pages
{
    public class CouponModel : PageModel
    {
        [BindProperty]
        public string Code { get; set; }
        public bool Succeeded { get; set; } = false;

        private static HttpClient httpClient = new HttpClient();

        public void OnGet()
        {
        }

        public void OnPost()
        {
            VerifyAsync().GetAwaiter().GetResult();
        }

        private async Task VerifyAsync()
        {
            Log.Logger.Information($"code={Code}");
            var res = await httpClient.GetAsync($"http://promoservice/api/Promo?code={Code}");
            var json = await res.Content.ReadAsStringAsync();
            Log.Logger.Information(json);
            var definition = new { verified = false , message =""};

            var result = JsonConvert.DeserializeAnonymousType(json, definition);
            if (!result.verified)
            {
                var e = new Exception(result.message);
                throw e;
            }

            Succeeded = result.verified;
        }
    }
}

GreeterService.cs

using System.Threading.Tasks;
using Grpc.Core;
using Microsoft.Extensions.Logging;

namespace RPCService
{
    public class GreeterService : Greeter.GreeterBase
    {
        public GreeterService()
        {
        }

        public override Task<HelloReply> SayHello(HelloRequest request, ServerCallContext context)
        {

            return Task.FromResult(new HelloReply
            {
                Message = Verify(request.Name)
            });
        }

        private string Verify(string code)
        {
            return "S";
        }
    }
}

PromoController.cs

using System;
using System.Threading.Tasks;
using Grpc.Net.Client;
using Microsoft.AspNetCore.Mvc;
using Microsoft.Extensions.Logging;
using RPCService;
using Serilog;

namespace PromoService.Controllers
{
    [Route("api/[controller]")]
    [ApiController]
    public class PromoController : ControllerBase
    {

        public PromoController()
        {
        }

        [HttpGet("Test")]
        public string GetTest()
        {
            return "Test succeeded";
        }

        [HttpGet]
        public async Task<VerificationResult> VerifyAsync([FromQuery]string code)
        {
            Log.Logger.Information($"Verification Request with code: {code}");
            AppContext.SetSwitch("System.Net.Http.SocketsHttpHandler.Http2UnencryptedSupport", true);

            var channel = GrpcChannel.ForAddress("http://rpcservice");
            var client = new Greeter.GreeterClient(channel);
            var reply = await client.SayHelloAsync(
                              new HelloRequest { Name = "VeifyClient" });
            
            if (reply.Message.StartsWith("S") == true)
            {
                return new VerificationResult { Verified = true };
            }
            else
            {
                return new VerificationResult { Verified = false, Message = "Invalid coupon code" };
            }
        }
    }

    public class VerificationResult
    {
        public bool Verified { get; set; }
        public string Message { get; set; }
    }
}