flutter how to create an dart:ffi struct reference

I created a struct with dart:ffi.

import 'dart:ffi';
import 'package:ffi/ffi.dart';

class TestStruct extends Struct{
   external Pointer<Utf8> strText;
   
   @Int32()
   external int nNum;

   @Bool()
   external bool bIsTrue;


   //contstruct
   TestStruct(String str, int number, bool state){
      strText = str as Pointer<Utf8>;
      nNum = number as int;
      bIsTrue = state as bool;
   }
}

I want to create a reference of TestStruct and use it. So I wrote the code.

TestStruct test = TestStruct("Text", 10, true);

but this is an error

Subclasses of 'Struct' and 'Union' are backed by native memory, and can't be instantiated by a generative constructor.  
Try allocating it via allocation, or load from a 'Pointer'.

I tried searching with the api documentation, but I didn't understand. Do you know how to create a struct as a reference?? thank you.

Example:

class InAddr extends Struct {

  factory InAddr.allocate(int sAddr) =>
      calloc<InAddr>().ref
        ..sAddr = sAddr;
        
  @Uint32()
  external int sAddr;
}

You can allocate this with calloc function

final Pointer<InAddr> inAddress = calloc<InAddr>();

And free the pointer with

calloc.free(inAddress);