OnActionExecutionAsync.cs

public async Task OnActionExecutionAsync(ActionExecutingContext context, ActionExecutionDelegate next)
{
    if (!context.ModelState.IsValid)
    {
        var errorsInModelState = context.ModelState
            .Where(x => x.Value.Errors.Count > 0)
            .ToDictionary(kvp => kvp.Key, kvp => kvp.Value.Errors.Select(x => x.ErrorMessage)).ToArray();

        var errorResponse = new ErrorResponse();

        foreach (var error in errorsInModelState)
        {
            foreach (var subError in error.Value)
            {
                var errorModel = new ErrorModel
                {
                    FieldName = error.Key,
                    Message = subError
                };

                errorResponse.Errors.Add(errorModel);
            }
        }

        context.Result = new BadRequestObjectResult(errorResponse);
        return;
    }

    await next();

    // after controller
}