https://stackoverflow.com/questions/76079895/how-to-give-currency-selection-dropdown-with-text-input-in-flutter

main.dart

import 'package:flutter/material.dart';
import 'package:pattern_formatter/pattern_formatter.dart';

void main() async {
  WidgetsFlutterBinding.ensureInitialized();
  runApp(MaterialApp(
    home: Scaffold(
      appBar: AppBar(
        title: const Text('Hello World'),
      ),
      body: const Center(child: CustomFormWidget()),
    ),
  ));
}

class CustomFormWidget extends StatefulWidget {
  const CustomFormWidget({super.key});

  @override
  State<CustomFormWidget> createState() => _CustomFormWidgetState();
}

const _supportedCurrency = {'₹': 100000, '\$': 10000, '€': 800, '￡': 600};

class _CustomFormWidgetState extends State<CustomFormWidget> {
  var _selectedCurrency = _supportedCurrency.entries.first.key;
  @override
  Widget build(BuildContext context) {
    final maxInput = ThousandsFormatter()
        .formatEditUpdate(
            const TextEditingValue(text: ''),
            TextEditingValue(
                text: _supportedCurrency[_selectedCurrency].toString()))
        .text;
    final dropdown = DropdownButton<String>(
      items: {'₹', '\$', '€', '￡'}
          .map<DropdownMenuItem<String>>(
            (e) => DropdownMenuItem(
                value: e,
                child: CircleAvatar(
                  backgroundColor: Colors.black,
                  child: Text(
                    e,
                    style: const TextStyle(color: Colors.white, fontSize: 24),
                  ),
                )),
          )
          .toList(),
      value: _selectedCurrency,
      icon: const Icon(
        Icons.keyboard_arrow_down,
        color: Colors.black,
        size: 32,
      ),
      onChanged: (value) {
        if (value != null) {
          setState(() {
            _selectedCurrency = value;
          });
        }
      },
    );

    return Column(
      mainAxisSize: MainAxisSize.min,
      mainAxisAlignment: MainAxisAlignment.center,
      children: [
        const Text(
          'Enter amount',
          style: TextStyle(fontSize: 24, fontWeight: FontWeight.w700),
        ),
        const SizedBox(
          height: 20,
        ),
        FractionallySizedBox(
          widthFactor: 0.8,
          child: TextFormField(
            maxLines: 1,
            decoration: InputDecoration(
              prefix: dropdown,
              contentPadding: const EdgeInsets.only(bottom: 10, right: 48),
            ),
            style: const TextStyle(fontSize: 32),
            textAlign: TextAlign.center,
            inputFormatters: [
              ThousandsFormatter(),
            ],
            validator: (value) {
              final newValue = value?.replaceAll(RegExp(r'^[0-9]'), '');
              final intValue = int.tryParse(newValue ?? '0');
              if (intValue != null &&
                  intValue <= _supportedCurrency[_selectedCurrency]!) {
                return null;
              }
              return 'Invalid Input';
            },
          ),
        ),
        const SizedBox(
          height: 10,
        ),
        Text(
          'Maximum of $maxInput',
          style: const TextStyle(fontSize: 22),
        )
      ],
    );
  }
}