Magento-1.7 Checkout Backend – Add New Field in Checkout

You can do this simply with some javascript. The javascript implementation can be done in a few ways. This is but one.

The quickest would be to copy the core template to your theme

/app/design/frontend/base/default/template/checkout/cart/coupon.phtml

and then set the coupon element to display:none with some in-line css (or you can adjust the carts stylesheet if you do not want to use in-line css, which is better)

Then all you have to do is place your 'click here' element to have a click event to toggle the coupon display and hide the click element.

Have not tested this code for 100% correctiveness

<form id="discount-coupon-form" action="<?php echo $this->getUrl('checkout/cart/couponPost') ?>" method="post">
    <a class="show-coupon-box" href="#">Have a coupon? Click to enter</a>
    <div class="discount" style="display:none">
        <h2><?php echo $this->__('Discount Codes') ?></h2>
        <div class="discount-form">
            <label for="coupon_code"><?php echo $this->__('Enter your coupon code if you have one.') ?></label>
            <input type="hidden" name="remove" id="remove-coupone" value="0" />
            <div class="input-box">
                <input class="input-text" id="coupon_code" name="coupon_code" value="<?php echo $this->escapeHtml($this->getCouponCode()) ?>" />
            </div>
            <div class="buttons-set">
                <button type="button" title="<?php echo $this->__('Apply Coupon') ?>" class="button" onclick="discountForm.submit(false)" value="<?php echo $this->__('Apply Coupon') ?>"><span><span><?php echo $this->__('Apply Coupon') ?></span></span></button>
                <?php if(strlen($this->getCouponCode())): ?>
                    &nbsp; <button type="button" title="<?php echo $this->__('Cancel Coupon') ?>" class="button" onclick="discountForm.submit(true)" value="<?php echo $this->__('Cancel Coupon') ?>"><span><span><?php echo $this->__('Cancel Coupon') ?></span></span></button>
                <?php endif;?>
            </div>
        </div>
    </div>
</form>

and some javascript to deal with the click

$$('.show-coupon-box')[0].observe('click', function(e){
   $$('.discount')[0].toggle(); $$('.show-coupon-box')[0].toggle(); 
   e.stop();
});

Please review belwo link;

https://devdocs.magento.com/guides/v2.0/howdoi/checkout/checkout_new_field.html

Thanks