There is no automatic linkage between the TurboTax desktop editions and the online editions, they are separate programs with totally separate price points.
You can download the 2017 tax data file from the online account to transfer to the 2018 desktop edition. See this TurboTax support FAQ for transfers using the TurboTax desktop editions for Windows - https://ttlc.intuit.com/questions/1901506-transfer-last-year-s-return-to-2018-turbotax-for-windows
