echo '<input list="clients" name="client"><datalist id="clients"
name="client" value="'.$_POST['client'].'">';

while($row=mysqli_fetch_array($result))
{
    if(isset($_POST['client']))
    {
        echo'<option value="'.$_POST['client'].'">';
    }
    else
    {
        echo'<option value="'.$row['partyname'].'">';
    }
    echo '</datalist>';
}

<form method="post">
   <?php

     $con = mysqli_connect('localhost','root','','test');
     $sql = 'select * from person';
     $result = mysqli_query($con,$sql);
     echo '<input list="clients" name="client">
     <datalist id="clients" name="client" value="'.$_POST['client'].'">';


      if(isset($_POST['client']))
      {
         echo'<option value="'.$_POST['client'].'">';

       }
      else
    {
        while($row=mysqli_fetch_array($result))
         {
             echo'<option value="'.$row['nameOfPerson'].'">';
           }
      }
        echo '</datalist>';
    ?>
     <br /><input type="submit" value="submit" name='submit'/>

   </form>