"CustomConfig": {
  "Setting1": "Hello",
  "Setting2": "Hello" 
}

public class CustomConfig
{
        [Required(ErrorMessage = "Custom Error Message")]
        public string Setting1 { get; set; }

        public string Setting2 { get; set; }

        public string Setting3 { get; set; }
}

namespace MvcTest
{
    public class Startup
    {
        public void ConfigureServices(IServiceCollection services)
        {
            services.AddOptions<CustomConfig>()
                    .Bind(Configuration.GetSection("CustomConfig"))
                    .ValidateDataAnnotations();
        }

namespace MvcHealthCheckTest
{
    public class Startup
    {
        public void ConfigureServices(IServiceCollection services)
        {
            services.AddOptions<CustomConfig>()
                    .Bind(Configuration.GetSection("CustomConfig"))
                    .ValidateDataAnnotations()
                    .Validate(customConfig =>
                    {
                        if (customConfig.Setting2 != default)
                        {
                            return customConfig.Setting3 != default;
                        }
                        return true;
                    }, "Setting 3 is required when Setting2 is present");
        }

<TargetFramework>netcoreapp2.2</TargetFramework>

<PackageReference Include="Microsoft.AspNetCore.App"  />

<configuration> 
  <system.webServer> 
    <handlers> 
     <add name="aspNetCore" path="*" verb="*" modules="AspNetCoreModuleV2" resourceType="Unspecified" /> 
    </handlers> 
    <aspNetCore processPath="dotnet" arguments=".\MyApp.dll" hostingModel="InProcess" /> 
   </system.webServer> 
</configuration>