How to prevent when unknown property insert in input API post request?

I'm working on Asp.net core and applying back-end validation using Fluent Validation.

The problem is, When I pass unknown property from postman over post request so it gives true response. I want to validate that unknown property.

Fluent Validation Code:

 public class ScheduleADemoValidator : AbstractValidator<ScheduleADemo>
{
    public ScheduleADemoValidator()
    {
        RuleFor(x => x.FirstName).NotEmpty().WithMessage("FirstName is required").MaximumLength(20);
        RuleFor(x => x.LastName).NotEmpty().WithMessage("LastName is required").MaximumLength(20);
        RuleFor(x => x.Iam).NotNull().Must(x => x == false || x == true);
        RuleFor(x => x.Iaminterestedin).NotEmpty().NotEqual("-None-").WithMessage("Iaminterestedin is required");
        RuleFor(x => x.Email).NotEmpty().WithMessage("Email is required").EmailAddress();
        RuleFor(x => x.Phone).NotEmpty().WithMessage("Phone is required").Matches("^([0-9]{3})?[-]([0-9]{3})?[-]([0-9]{4})$"); 
        RuleFor(x => x.NABP).NotNull().Matches("^([0-9]{7})$");
        RuleFor(x => x.Pharmacyname).NotEmpty().WithMessage("Pharmacyname is required").MaximumLength(20);
        RuleFor(x => x.Legalbusinessname).Length(0, 20);
        RuleFor(x => x.Address).MaximumLength(50);
        RuleFor(x => x.City).MaximumLength(50);
        RuleFor(x => x.State).NotEmpty().NotEqual("-None-").WithMessage("State is required").Length(2);
        RuleFor(x => x.ZipCode).Matches("^([0-9]{5})$");
        RuleFor(x => x.IheardofPrimeRxfrom).NotEmpty().NotEqual("-None-").WithMessage("IheardofPrimeRxfrom is required").MaximumLength(30);
        RuleFor(x => x.Iamlookingtobuywithin).NotEqual("-None-").WithMessage("-None- is not accepted").MaximumLength(30);
        RuleFor(x => x.Iwouldliketoseeademofor).NotEmpty().NotEqual("-None-").WithMessage("Iwouldliketoseeademofor is required").MaximumLength(30);
        RuleFor(x => x.Message).NotNull().MaximumLength(50);
    }
    protected override bool PreValidate(ValidationContext<ScheduleADemo> context, ValidationResult result)
    {
        if (context.InstanceToValidate == null)
        {
            result.Errors.Add(new ValidationFailure("", "Iam must be true or false"));
            return false;
        }
        return true;
    }
}

My controller code:

 if (!validationResult.IsValid)
            {
                return Json(new { failure = validationResult.Errors.First().ErrorMessage });
            }
            var _savingScheduleADemo = new SavingScheduleADemo(_contentManager, _contentItemDisplayManager, _updateModelAccessor, _contentDefinitionManager, _session, _idGenerator);
            var result = await _savingScheduleADemo.CreateScheduleADemo(
                scheduleADemo.FirstName,
                scheduleADemo.LastName,
                scheduleADemo.Iam,
                scheduleADemo.Iaminterestedin,
                scheduleADemo.Email,
                scheduleADemo.Phone,
                scheduleADemo.NABP,
                scheduleADemo.Pharmacyname,
                scheduleADemo.Legalbusinessname,
                scheduleADemo.Address,
                scheduleADemo.City,
                scheduleADemo.State,
                scheduleADemo.ZipCode,
                scheduleADemo.IheardofPrimeRxfrom,
                scheduleADemo.Iamlookingtobuywithin,
                scheduleADemo.Iwouldliketoseeademofor,
                scheduleADemo.Message);
            if (result)
            {
                return Json(new { success = true });
            }
            else
            {
                return Json(new { success = false });
            }
        }

Response on Postman over unknown field

Please give me your kind advice.