Good morning, @mateen.
I'm happy to provide information about adding custom fields to a bill.
With QuickBooks Online the feature is not available.
Although, you can add a memo to the bill that you are creating.
Here's a quick look:
I'm also going to leave feedback about this so we can review this for a possible update in the future.
If you need help with anything else please feel free to reach back out to the Community. Have a great week!