Just had a look, there is no one-click function or? Not even in Wallet.

It would also be absolutely not reprehensible if Leo would let newly created accounts automatically vote for their own Witness. And if not that, at least a pop-up on the Xth login or something. But to the new user who comes to LeoFinance via LeoFinance.... Well, he will probably be able to live with voting for the Witness of LeoFinance, even if he doesn't know about it.