Since Bigcommerce doesn't have a root folder access to install the Service Worker file which is needed in order to have the automatic native-browser opt-in, you can still the basic copy and paste of script method, but the key is you'll need to change from HTTPS to HTTP within your Subscribers dashboard which is a bit weird but that basically enables that method for the installation.
Now, please scroll down on your account dashboard and navigate to "CODE INSTALL"> under Manual installation, you'll find your installation code to paste to your website admin.
Now to time go to your BigCommerce platform to install your script.
Please see this installation guide to BigCommerce.
-Log in to your BigCommerce dashboard and navigate to "STOREFRONT" and then "SCRIPT MANAGER". It should look something like this:
-Press the "SAVE" button on the bottom right corner of the "Create Script" page:
-Press the "Create Script" button in the upper right corner.
-Enter "Subscribers" in the "Name of Script" field
Leave "Description" blank
Choose "Footer" for "Location on Page"
For "Select Pages where script will be added", you can choose "All Pages" if you want your Subscribers push notification to appear on any page on your site (this will allow you to use the targeting feature inside of Subscribers). Or you can choose any of the other options like "Store Pages" or "Checkout" if you're only planning to use Subscribers on those pages. It's totally up to you.
For "Script Type", choose "Script". This one is important!
Paste the script that you had previously copied from your Subscribers dashboard into the last field on this page, which is called "Script Contents"
Now go back to your Subscribers dashboard and proceed with the verification.
Please note that our system may recognize some errors with the installation but once you confirm that the installation is done correctly, you may click on the "Override verification" regardless of what error messages are showing.