The account-wide setting to create new when adding a customer will use the naming rules provided in the same config section and create new customer entries in QuickBooks Online when a new customer is created in Stax Bill. After that, it will sync any new transactions associated with it.
When creating a new customer, in the form, there is a section dedicated to whether or not to override the default behaviour (but not name) for QuickBooks Online:
To manage it once the customer has been created, but is not linked:
If the customer is not yet active, the option on what to do can be changed a little:
And it will perform the selected action if the options are saved and when the customer is activated.
If the customer is active, the screen changes slightly to show what actions can be taken. 
When Connect Customer is clicked, another prompt will ask whether to latch to an existing customer or create a new one: 
If Create New is selected, the Next prompt will be to ask how far back to sync existing transactions. 
If "Latch to existing customer" is selected as Create new, the next prompt will be a customer search, and the following will be the prompt for how far back to send the transactions.
Linking individual customers to Quickbooks Online
