using System;

​

public class Program

{

    public static void Main()

    {

        var _requestData = "username=test01&password=aaaaaa&otp=123456&__RequestVerificationToken";

        var startIndex = _requestData.IndexOf("password=");

        if (startIndex > -1)

        {

            var endIndex = _requestData.IndexOf("otp=");

            if (endIndex > -1 && endIndex > startIndex)

            {

                Console.WriteLine(startIndex);

                Console.WriteLine(endIndex);

                startIndex = startIndex + "password=".Length;

                _requestData = _requestData.Remove(startIndex, endIndex - startIndex);

                _requestData = _requestData.Insert(startIndex, "********");

            }

        }

​

        Console.WriteLine(_requestData);

    }

}