Can wix prompt for a windows service username/password?

Yes, but it's not built-in. You can setup a GUI in WiX (which is not too intuitive) where you can prompt for any setting you'll need in later steps.

First, check out how to create a GUI that sets custom properties by following these guidelines, ensuring that your GUI fills properties SERVICEACCOUNT and SERVICEPASSWORD. Then use the ServiceInstall element with those properties as shown here.

WiX doesn't (or at least didn't) have anything nice out of the box for what you are looking for. Best option that I am aware of is to roll-your-own.

https://www.geekproject.com/post/wix-service-account-dialog/

The Community MSI Extensions on GitHub contains a ServiceCredDlg dialog for prompting username and password. According to the docs, it can test the credentials are valid and that user has 'logon as a service' rights.