How to Assign Countries at Turn X?

Hi, just wondering what's the best way to give a player additional land at a later turn without allowing any possibility of the opposing player taking that land if they attack it on the same turn.

Is dropping the units directly onto the city the right move?

If the country is neutral, you should add the player units as an event on its capital.
The tricky part is that you can't add events on capital cities, so you have to add the events first then turn the city into capital afterwards.
Also make sure to add a special neutral unit there to prevent other players from capturing it before the event, otherwise they'll become the owners of the event units.