if the page is opened so count and put it to the value of hidden input code example

Example 1: input hidden selected

<form>
    <select id="dropdown" name="dropdown" onchange="changeHiddenInput(this)">
        <option value="j.hotmail.com">Jens</option>
        <option value="a.hotmail.com">Adam</option>
        <option value="d.homtail.com">Dan</option>
    </select>
    <input type="hidden" name="hiddenInput" id="hiddenInput" value="" />
 </form>

    function changeHiddenInput (objDropDown)
    {
        var objHidden = document.getElementById("hiddenInput");
        objHidden.value = objDropDown.value; 
    }

Example 2: if the page is opened so count and put it to the value of hidden input

<form>
  <div>
    <label for="title">Post title:</label>
    <input type="text" id="title" name="title" value="My excellent blog post">
  </div>
  <div>
    <label for="content">Post content:</label>
    <textarea id="content" name="content" cols="60" rows="5">
This is the content of my excellent blog post. I hope you enjoy it!
    </textarea>
  </div>
  <div>
    <button type="submit">Update post</button>
  </div>
  <input type="hidden" id="postId" name="postId" value="34657">
</form>

Example 3: if the page is opened so count and put it to the value of hidden input

html {
  font-family: sans-serif;
}

form {
  width: 500px;
}

div {
  display: flex;
  margin-bottom: 10px;
}

label {
  flex: 2;
  line-height: 2;
  text-align: right;
  padding-right: 20px;
}

input, textarea {
  flex: 7;
  font-family: sans-serif;
  font-size: 1.1rem;
  padding: 5px;
}

textarea {
  height: 60px;
}