In CST you'll use a discrete port. You select points in both ends (by using the pick tools or the "p" shortcut). Once the two extreme points are selected the discrete port will be automatically placed. Choose for your case a 50 Ohms.

If the ground plane is wide and you cannot pick a specific point, just use the shortcut "f" (from "pick face") on the ground face. CST will automatically choose the shortest path between the antenna and the ground plane.

Take a look into the attached picture. There you use the "e" to pick an edge, which provides even better results, since the port inductance will be smaller in comparison to the port with 2 points.

Good luck! 

Thanks Dr. Marcelo, this was a helpful information, I tried it and got a resonance in a different frequency rather than in HFSS !

I don't know the problem with the design 

Have you checked your Mesh cells?

Are boundary conditions defined correctly?

Mousa

In case of the Frequency solver, results from CST and the other HFSS are exactly the same, usually. Maybe as Mr. Foudazi remarked, there might be something else related to the electromagnetic setup.

Thanks Gents , Ya it seems it is a setup problem, not a port issue .

I will dig deeper in the design again .

Thanks for your appreciated help