How to show widget or text right after value?

suffixText set at the right of the TextField's container by default. Try below code with custom textfield with currency at right of the value with onTextChanged method.

class CTextField extends StatefulWidget {
  @override
  CTextFieldState createState() => CTextFieldState();
}

class CTextFieldState extends State<CTextField> {
  TextEditingController controller = TextEditingController();

  @override
  void initState() {
    super.initState();
    controller.addListener(onTextChanged);
  }

  @override
  void dispose() {
    controller.removeListener(onTextChanged);
    controller.dispose();
    super.dispose();
  }

  void onTextChanged() {
    String text = controller.text;
    if (!text.endsWith(' T')) {
      controller.value = TextEditingValue(
        text: text.replaceAll(' T', '') + ' T', 
        selection: TextSelection.collapsed(offset: text.length + 4),
      );
    }
  }

  @override
  Widget build(BuildContext context) {
    return TextField(
      controller: controller,
      keyboardType: TextInputType.number,
      decoration: InputDecoration(
        labelText: 'Amount',
        hintText: 'Value',
      ),
    );
  }
}

Another way with Container:

         Container(
              decoration: BoxDecoration(borderRadius: BorderRadius.circular(5)),
              child: Row(
                mainAxisAlignment: MainAxisAlignment.spaceBetween,
                children: [
                  Row(
                    children: [
                      IntrinsicWidth(
                        child: TextField(
                          decoration: InputDecoration(
                            border: InputBorder.none,
                            hintText: '25',
                          ),
                        ),
                      ),
                      Text(
                        'T',
                      )
                    ],
                  ),
                  Icon(Icons.arrow_right_sharp)
                ],
              ),
            ),

TextField(
  controller: _controller,
  decoration: InputDecoration(
    labelText: 'Enter Amount',
    border: OutlineInputBorder(),
    suffixText: ' USD', // Currency symbol as suffix
    suffixStyle: TextStyle(fontSize: 16, fontWeight: FontWeight.bold),
  ),
  keyboardType: TextInputType.numberWithOptions(decimal: true),
),