Set Default Income/Expense Account

You can assign a specificincome account for products and services.

To do so:

1. Click the Gear icon, then choose Products and services.
2. Select Edit just beside the product or service or click New.
3. Scroll down and under Income Account drop-down, select the appropriate account.
4. Click Save and close.

Feel free to check this article for reference:Use Products and Services in QuickBooks Online.