You're welcome! There isn't anything that's built into the product at this time that would autofill a sales rep based on the customer you choose. Using the method you've got is the workaround I would suggest, especially if you'd like your customer to be able to see the rep's name as well.
Another option to consider is class tracking. This is available in QuickBooks Online Plus package accounts and still requires you to manually choose an option, but the classes can be pre-set so that you don't have to type something every time. Learn more about class tracking here: Get started with class tracking in QuickBooks Online. Essentially, you could set up each sales rep as a "class," and then add their name to sales form. This is only for internal tracking, however, so your customer wouldn't be able to see the name.
You can also take a look at available partner applications to see if something there can do what you're looking for. To check the options, click the Apps tab in your account and then use keywords to search the options.
I hope that helps!
