If a client automatically takes a discount for fast payment how do I enter the credit?

Hi there, @accounts225.

You'll want to enter the discount amount in the discount field. Here's how.

In your QuickBooks Online (QBO):

1. Go to the Gear icon.
2. Select Account and Settings.
3. Go to the Sales tab and select Sales from content section.
4. Mark Discount to turn on the discount feature, click Save, and Done. 
5. Once done, go to the (+) Plus icon.
6. Select Invoice.
7. Fill in the details and select the Discount value from the drop-down menu and enter the discount amount. 
8. Click Save and close or Save and send.

You can also consult with your accountant if you want more tips on how to better record this. 

Then, in case you'll need to enter a refund to your customer, you can always check this article for reference: Enter and apply credit notes and delayed credits in QuickBooks Online.

I'll be around if you have other questions or concerns. I'd be glad to help you. Take care!