To include a one-time charge or discount to the first subscription invoice, you can do so by adding an invoice item to the customer before creating the subscription. This can be done using the API or directly in the Stripe Dashboard.
Example code for adding an invoice item using the API can be found in our Subscription Invoices documentation.
To add an invoice item in the Dashboard:
Find and click on the customer in your Customers list.
Under Pending Invoice Items, click + Add invoice item.
Add the description, amount, and currency, and select Auto (next to be billed) from the Subscription dropdown.
Click Create invoice item.
From there, add the subscription to the customer, and the invoice will generate for the cost of the subscription and the additional pending invoice item.