public class ExampleInput
{
  public string Example { get; set; }
}

public class ExampleInputValidator : AbstractValidator<ExampleInput>
{
  public ExampleInputValidator()
  {
    RuleFor(input => input.ExampleProperty)
      .NotEmpty()
      .WithMessage("Property is empty");
  }
}

# Since 10.2.0 https://github.com/FluentValidation/FluentValidation/releases/tag/10.2.0
services.AddFluentValidation();

services.AddGraphQLServer()
  .AddFluentValidation();

descriptor.Field(x => x.Example(default!))
  // Explicit over implicit preferred
  // You have to add .UseFluentValidation()/attribute to all arguments requiring validation
  .Argument("input", argument => argument.UseFluentValidation());

... Example([UseFluentValidation] ExampleInput input) { ... }

services.AddGraphQLServer()
  .AddFluentValidation(options =>
  {
    options.SkipValidation(...)
      .UseErrorMapper(...)
      .UseInputValidators(...);
  });

descriptor.Field(x => x.Example(default!))
  .Argument("input", argument => argument.UseFluentValidation(options =>
  {
    options.SkipValidation(...)
      .UseErrorMapper(...)
      .UseInputValidators(...)
      .UseValidationStrategy(...)
      .UseValidator<ExampleInputValidator>()
      .UseValidator<ExampleInput, ExampleInputValidator>()
      .UseValidator<ExampleInput, ExampleInputValidator>(strategy =>
      {
        strategy.IncludeProperties(input => input.ExampleProperty);
        // ...
      });
  }));

... Example([UseFluentValidation, UseValidator((typeof(ExampleInputValidator))] ExampleInput input) { ... }