Welcome to the Community, @Pranav1958.

I can help you add the bank account details to an invoice in QuickBooks Online.

Currently, the option to enable a default field for the bank account details is unavailable. As a workaround, I recommend adding a custom field for the information you want to show in the sales form.

However, you may need to enter the account details manually during the process. Here's how:

1. Click the Gear icon in the upper-right corner.
2. Select Custom form styles under Your Company.
3. Choose a template, then press Edit below the Action column. 
4. Go to the Content tab.
5. Click the pencil icon at the top-right corner. 
6.  Press +Custom field under Display. 
7. Enter "Bank account" as the custom name. 
8. Hit Done.
9. Manually enter the account number when creating an invoice. 

Additionally, I recommend the following resources, which contains information about customizing transactions, as well as the email content of the invoices:

• Customise invoices, estimates, and cash memos in QuickBooks Online
• Customise your invoice emails

If you have additional questions or concerns while working in QBO, leave a comment below. I’ll be happy to help you out.