Head over to "Send to Customer" section of app. Note: you must have pro or higher plans to use this feature.
Click Invoice
Under the "drafts" section, you'll find code to copy and paste. This will be visible to you once you upgrade to paid plans.
This will look something like this :
Click "copy".
Head over to Shopify setting and under Notifications section. Click on "Draft Order Invoice"
paste the code and click "preview" to see how it's displayed on Draft order confirmation email. Click "Save" when done.