Создание динамических ссылок с помощью JQuery — индивидуальный идентификатор сообщения

$('blockquote').each(function(blockquote){
  // Find the <cite> element.
  var _cite = $(this).find('cite');
  // get the first .post_id's id attribute value.
  var _postId = $(this).find('.post_id').first().attr('id');
  console.info(_postId); // <-- just a side note. This will be a string. If you need to use such id, remember to make it numeric if you need to use it as a number. You can do that by doing Number(_postId) or, simply, +_postId.
  // Generate the new html by generating a new anchor with the desired link.
  var newHtml = `<a href = "/post-${_postId}.html">${_cite.html()}</a>`;
  // Override the <cite> html.
  _cite.html(newHtml);
});

<script src = "https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
<blockquote>
  <cite>Testuser said:</cite>
    <div>This is a test</div>
    <span class = "post_id" id = "123"></span>
</blockquote>

<blockquote>
 <cite>Testuser2 said:</cite>
  <div>This is a test</div>
  <span class = "post_id" id = "1234"></span>
</blockquote>

<blockquote>
 <cite>Testuser3 said:</cite>
   <div>This is a test</div>
   <span class = "post_id" id = "12345"></span>
</blockquote>

var elements = document.querySelectorAll('.post_id');

elements.forEach(e => {
  var testUserID = e.getAttribute('id')
  var newlink = document.createElement('a');
newlink.setAttribute('class', 'post');
newlink.setAttribute('href', 'http://example.com/post-'+testUserID+'.html');
  newlink.innerHTML = 'test';
  
  
  var cite = e.parentNode.querySelector("cite")
  cite.appendChild(newlink)
})

<blockquote>
  <cite>Testuser said:</cite>
    <div>This is a test</div>
    <span class = "post_id" id = "123"></span>
</blockquote>

<blockquote>
 <cite>Testuser2 said:</cite>
  <div>This is a test</div>
  <span class = "post_id" id = "12222234"></span>
</blockquote>

<blockquote>
 <cite>Testuser3 said:</cite>
   <div>This is a test</div>
   <span class = "post_id" id = "12345"></span>
</blockquote>