add url before output taken from javascript

I want to add a URL before the javascript output, basically, I want to enter the URL from the input field and my javascript convert it into Base64 and show it in the output box but problem is that I want to add URL as default before my base64 encoded code. Like below

User input: https://google.com
User Output (Base64): asdweoiuweosdj

But I want this

User Input:  https://google.com
User Output: https://example.com?=asdweoiuweosdj

<!DOCTYPE html>
<html>

<body>

  <div id="container">
    <div class="IO-field">
      <input type="text" id="plain-text">
      <button type="button" onclick="getInputValue();" id="text-submit">click</button>
      <input class="text" id="base-64">
    </div>


    <script>
      var plainTextIOField = document.querySelector("#plain-text"),
        base64IOField = document.querySelector("#base-64"),
        textSubmit = document.querySelector("#text-submit"),


        //use the index of each character in the array as the key that links value and corresponding char in base64 table  
        base64Table = ["A", "B", "C", "D", "E", "F", "G", "H", "I", "J", "K", "L", "M", "N", "O", "P", "Q", "R", "S", "T", "U", "V", "W", "X", "Y", "Z",
          "a", "b", "c", "d", "e", "f", "g", "h", "i", "j", "k", "l", "m", "n", "o", "p", "q", "r", "s", "t", "u", "v", "w", "x", "y", "z",
          "0", "1", "2", "3", "4", "5", "6", "7", "8", "9", "+", "/"
        ];


      //function that converts a standard ASCII character decimal value into its binary correspondant and turns it into a string
      function dec2bin(dec) {
        var bin = (dec >>> 0).toString(2);
        if (bin.length < 8) {
          var itrs = 8 - bin.length;
          for (var i = 0; i < itrs; i++) {
            bin = "0" + bin;
          }
        }
        return bin;
      }


      textSubmit.addEventListener("click", function(e) {

        //block browser form reloading the page
        e.preventDefault();

        //declare variables needed during the conversion
        var string = plainTextIOField.value,
          stringToArray = string.split(''),
          s = "",
          array64 = [];

        //for each letter in the stringToArray array get its charCode, convert it to binary form, make it a string and concatenate it with the s string
        for (var i = 0; i < stringToArray.length; i++) {

          var charCode = stringToArray[i].charCodeAt(0);
          s += dec2bin(charCode);

        }

        //make s an array made of each bit represented in the s string
        s = s.split('');


        //put all the strings of the s array inside array64 and separate each series of 6 consecutive elements with a single whitespace
        for (var i = 0; i < s.length; i++) {

          if (i > 0 && i % 6 === 0)
            array64.push(" ");
          array64.push(s[i]);

        }

        //concatenate all of array64's elements into a single string and then break the string using the whitespaces just added as divider
        array64 = array64.join('').split(' ');


        //make sure each string representing a binary value in array64 is 6 chars long
        if (array64[array64.length - 1].length < 6) {

          var array64Last = array64.pop(),
            nOf0s = 6 - array64Last.length;

          for (var i = 0; i < nOf0s; i++) {

            array64Last += "0";

          }

          array64.push(array64Last);

        }


        //make sure the array64's length is a multiple of 4, and if not add correct padding as base64 encoding requires
        if (array64.length % 4 !== 0) {

          var padding = 4 - (array64.length % 4);

          for (var i = 0; i < padding; i++) {

            array64.push("=");

          }

        }


        //substitute each string in array64 with its corresponding binary value and then with the value get the right character from the base 64 table
        for (var i = 0; i < array64.length; i++) {
          if (array64[i] == "=") {
            continue;
          }
          array64[i] = base64Table[parseInt(array64[i], 2)];
        }

        //concatenate all the characters inside of array64 and done :D
        base64IOField.value = array64.join('');

      })
    </script>

</body>

</html>

Please Help I am very confused about it. I am a beginner in javascript