It seems to be pretty simple,
The default e-mail template that is sent by Magento is:
- Customer: vendor/magento/module-sales/view/frontend/email/order_new.html
- Guest: vendor/magento/module-sales/view/frontend/email/order_new_guest.html
You must not edit those files directly on vendor. They can be loaded on admin panel:
Marketing > Communications > Email Templates;
Click on "Add New Template" button;
Load the default template (Order or Order Guest);
Include a title for the new template;
Include the link on the shipment section;
Save the template;
Go to Stores > Settings > Configuration and then Sales > Sales Emails
On the order section you must deselect the option Use system value
and then choose the template that you have created.