Hello and welcome to the Community, office-msltransp
It isn't possible to assign an account type/nominal code directly to a customer invoice in QuickBooks Online.
Instead, the account type is linked to the product/service selected on the item line. You can manage the accounts linked to your products & services by going to Sales > Products & services > Edit > See the Income account field.
When you have entered and saved the invoice, you can check the account types that it has posted to by clicking More on the invoice footer > Transaction journal.
Thanks again contacting Intuit support today. I'm just a post away if you require additional assistance.
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