add a text field to the checkout form on the checkout page:
<input name="rewardsCard" type="text" id="rewardsCard" value="" />
On the confirm page, add a hidden form element of the same name to capture the value posted from the checkout page:
<input name="rewardsCard" type="hidden" id="rewardsCard" value="<?php echo((isset($_POST["rewardsCard"]))?$_POST["rewardsCard"]:"") ?>" />
In your database, edit the order table, create a new column to capture the reward number entered.
On the confirm page, edit the eCart Store Order Summary server behavior. Bind the new rewards Card database column to the rewardsCard form element.