If you want the customer to be able to activate the certificate in the shopping cart, you need to make sure that in the page Shop - Plugins - Gift certificates the corresponding setting is enabled.
You can always change the General form output template.
If you need to change the output template for a particular storefront, you can go to the Shop - Storefront - Templates and create the file
giftcertificates.form.html
Copy the standard output template to the contents of this file and change it as you need.
The place of the form
If you use a one-step checkout, the form will be embedded automatically in the desired location.
When you are using multi-step checkout, hook frontend_cart is used by default:
<!-- plugin hook: 'frontend_cart' -->
{* @event frontend_cart.%plugin_id% *}
{foreach $frontend_cart as $_}{$_}{/foreach}
You can change the output place of the form.
Add construction in the template cart.html:
{if $wa->shop && method_exists('shopGiftcertificatesPluginHelper', 'getCartForm')}
{shopGiftcertificatesPluginHelper::getCartForm()}
{/if}