Hello, hdsds19.

Thanks for getting back to us! I have an idea on where you can put your customer's account number in their profile. 

You might have come across this workaround, but you can add the customer number in the Company box. This box works best since it appears together with the address and complete customer's name (without overlapping any data). Here's an example: 

You don't need to add the customer number in the Print on Check as box (or anywhere where there's already an important data). There are several fields that you can use as well. Try adding it in Other or Notes. When integrating QuickBooks with your CRM, map these fields accordingly. 

A sample check preview:

An invoice sample: 

If you need to do other tasks in QuickBooks, our articles can help you out. Look for them in our Help Articles page.

Change the flag at the top header beside Go to QuickBooks.com if you're using a version in a different country. 

I'm always here to help if you have future questions about managing your customers. I also welcome any other concerns you might have for QuickBooks Online. Just let me know in the reply section below, and I'll get back to you as soon as possible. Take care out there.