Добавете скритото поле:
на 1.php
echo '<form style="display:none;" name="answer_'.$index.'" method="post" action="output.php">'; // I dont think openning form from row to row would be nice!
echo '<input type="hidden" name="questionid" value="<?php echo $row[ID]?>"/>'
echo '<textarea type="text" class="addtext" name="addtext" required id="addtext_'.$index.'" placeholder="Please type your answer here.." ></textarea>';
echo '<button onClick="addsubmit('.$index.');" type="submit" id="addsubmit_'.$index.'" class="addsubmit"><B>Submit</B></button>';
echo '</form>';
на страница 2.php
$qid = $_POST['questionid']; //get the question id and insert in the table
