How do you add a customer link to a bill

Hello there, @dvande. Note that you're not doing wrong. Turn on the billable expense feature first to achieve your goal. I know how to do this. Let me help you.

Turning on the billable expense is recommended to add a customer to a bill. Here's how:
 

1. In Settings, click Account and Settings.
2. Hit the Expenses tab.
3. Go to the Bills and Expenses section to choose Edit.
4. Turn on the following:

• Show Items table on expense and purchase forms
• Track expenses and items by customer.
• Make expenses and items billable.

5. You can also set up the Markup rate, Billable expense tracking, and Sales tax charge.
6. Enter bill payment terms.
7. Once done, tick Save and Done.

However, I suggest upgrading your current subscription if you don't have the turn-on option. The billable expense is only available in QuickBooks Online Plus and Advanced. For a guide to upgrading, check out this link: Upgrade or downgrade your QuickBooks Online edition.

Moreover, to help you handle any reports you want to pull up in QBO, see this article for additional help: Customize reports in QuickBooks Online.

Reply to my post if you need more answers to any of your queries. I'm more than happy to assist.