How to add order number on invoice for customer reference?

Hello there, @theuns@stopdrip.

Adding an order number on your invoice template is just a breeze. I've got the steps that you need to accomplish this goal.

You can add a custom field on your invoice template so you can enter the order number for your customer's reference. Know that this option is only available to the  QuickBooks Online Essentials or Plus versions.

To do that:

1. Click on the Gear icon.
2. Tap on Custom form styles under Your Company. 
3. Choose the template that you want to Edit or select on Invoice under New style if you want to start a new one. 
4. Refer to the Content tab and click on the header portion of the sample template. 
5. Under the Display column, click on Custom field.
6. Put a check mark to one of the custom field and enter Order Number. 
7. Press on Done.

I've attached a sample snip on what your invoice should look like after the customization:

You can always check out this link to give you more insights about adding custom fields on your sales templates in QuickBooks Online: Set up custom fields to appear on sales forms.

I'm just a reply away if there's anything else that you need. I'll make sure you're taken care of. Have a nice day!