How to add a custom field in opencart 2.1 checkout page

Hi, how do I add an extra custom field in my checkout page? Need it urgently. Am using oc version 2.1.0.2. My customers use third party payment methods and I would like a text box where they could type their payment transaction code. The text box should preferably appear after the customer clicks the checkout button but before they click on confirm order, in that if they don't fill in their transaction code they can't confirm the order. Please help me add this function. Thanks

If you need this urgently, your best bet is posting this in the Commercial Support Forum and paying a developer to build it for you.