Hi Susan, you can't setup a default tax code for a nominal code unfortunately.

However you can assign a default tax code for a customer or supplier account so if you are looking to just post a certain tax code for a specific task, i would advise just setting up a new customer/supplier account you post your task through each time with that specific tax code.

I (for example), have a supplier account setup for HMRC where i post all my HMRC transactions & the default tax code i have setup for that is T9.

Hope that helps.