How to make the tank siege?

There's ways to con it. It's always simpler just to remove the unit and replace it with a siege moded tank unit (which is technically considered a different unit to the game/editor).

I couldn't find any reliable way to control whether the AI would siege or unsiege a tank.

Unsiege isn't that hard: Just attack it from within the minimal range of the siege cannon. But forcing a siege reliably is a problem.

I think your best shot would be to create them sieged and then never perform any actions on them, except unsiege them with the method described above. They would always return to their point of creation though.

You can also unsiege by using Move Unit on the Siege Mode, iirc.

actually I need the unsieged tank to siege

A tank's normal behavior is to siege immediately if it has no orders and there aren't any non-invincible hostiles right next to it.
This behavior is lost if you change the tank's owner however, so it will never work for a player-owned tank.

For an AI-controlled tank that's moving around and fighting, just make all units around it invincible for a short time, it will stop fighting and siege, you can also kill/remove the units right next to it so it won't unsiege to try and attack them.