You can do it in your return without having to create a dummy account, although you can do that too. When you have your return open, at the top, click on:
Personal > Other Tax Situations > Other Tax Forms > Form W-4 and estimated taxes. Follow the prompts and TurboTax will ask for a variety of income & deduction information before arriving at an estimate of 2017 taxes.
And since you are using a desktop version, you can create and save as many dummy returns as you like. From the 'File' menu in the top left, just select 'New Tax Return'. Just make sure to save each return with a different name so you do not overwrite anything.
