How to assign a short point (ideal ground) in CST transient

Hi everyone!
In CST transient simulation, I have some question.
When I want to do transient simulation using CST, I want to assign one side of structure as a short point to the ideal ground.
It would make the low impedance current path to the ideal ground.
But, I don't know how to realize it.
there is some boundary condition. open, E=0, H=0, and so on...
When I simulate in frequency domain, i usually assign the open space as a boundary condition.
But, i understand it is not ground just adding vacuum in transient simulation. am i right?
Please help me. thank you.

Physically, there is no global ground. Ground is what you call ground, where you place your port's reference pin. However, even if you use PEC (perfect electric conductor) two points at some distance will have (and must have) delay and inductance between them.