Currently discounts created in the Ongoing app apply to all future recurring orders.
The ability to create a discount for only the 1st order is a new feature coming soon.
Here's a workaround.
To create a discount on a subscription for the first order only, you accomplish this with Shopify Discounts.
From Shopify, click Discounts
Click Create a Discount Code
Select from Types and enter the discount amount
Make sure to apply it to subscriptions
For the Recurring Payments for Subscriptions, choose "Limit discount to the first payment"
Review the summary and click Save.
Make sure to run a test order and see how the subscription discount works on your current subscription product, especially if you already have an automatic subscription discount that you created on Ongoing on the subscription plan.
To let your customers know about this discount code, you can email it, place it in the product description, and also add an announcement banner to your website.
Note: Automatic discounts do not work with Shopify Subscriptions. However, Discount codes do work with Shopify subscriptions.