It is not possible to choose which Seasonal Kingdom you will be placed in. When you sign up for the Seasonal Kingdom, you will be placed in one of the new Seasonal Kingdoms. This placement cannot be changed. You can, however, interact with other Seasonal Kingdoms through Ayrin's avatar or during the Clash of Kingdoms.

Players from the same Kingdom will be placed in the same Seasonal Kingdom.

Please note that your Seasonal Kingdom is assigned when the tournament starts, not when you create your seasonal progress. This means that if you change your Kingdom after the Seasonal Kingdom starts, this won't affect your placement in the Seasonal Kingdom. 

 

If you want to play alongside players who are in different Kingdoms, it is worth considering teleporting before the start of the event.