There's no provision for doing this within TurboTax. If you don’t purchase Will Builder before filing your return with TurboTax, you’ll have to reach out directly to ARAG, Will Builder's provider, using their Contact Customer Care page. They may not honor the offer that's available in TurboTax.
See the TurboTax Help article What's Will Builder by ARAG® in TurboTax? for product details.