Thanks for joining us here in the Community, info-sainsagenci.
I know it saves much of your time to automatically assign a sales rep to a customer when creating invoices. Currently, this option isn't possible yet. For now, you'll need to manually assign each time you create an invoice.
Our product development team never stops enhancing the software. With this, I recommend submitting a product suggestion inside QuickBooks. Your idea can help improve our product and I know a lot of users can take advance of it once it's approved.