If you are using BigCommerce you have one of the best e-commerce platforms.
Now you can have a free WhatsApp Widget for your website.
Create your account for free and personalise your Widget.
Then you need to copy and paste the code in your Website.
If you need to edit your header section you need to go to > Templates > components > common >
There you can find your header template and edit the file.
You will be able to copy the script you just generate with your free account.