function copyValue(id) {
    var dropboxvalue = document.getElementById('mydropbox').value;
    document.getElementById('mytextbox').value = dropboxvalue;
}

<td align="center"><input type="submit" name="insert"  onclick="copyValue(<?php echo $r['so_id'] ?>)" class="btn btn-success" value="Insert" /></td>
      <td><input class="form-control" name="dar_numberr" id="mytextbox" type="text" required="required" placeholder="Delivery Date" value="<?php echo $r['dar_numberr'] ?>"></textarea></td>

<?php
            $databaseHost = "localhost";
            $databaseUser = "root";
            $databasePassword = "";
            $databaseName = "csl_otd";

            $con=mysql_connect($databaseHost ,$databaseUser ,$databasePassword )or die ('Connection Error');
            mysql_select_db("csl_otd",$con) or die ('Database Error');
         ?>
             <select name="dar_number" id="mydropbox" class="form-control">
             <option value=""> -----------Available DAR Number----------- </option>
         <?php
            $dd_res=mysql_query("Select DISTINCT dar_number from dar");
            while($r=mysql_fetch_row($dd_res))
            {
               echo "<option value='$r[0]'> $r[0] </option>";
            }
         ?>
            </select>