The following image shows a stripline. The ground planes are defined with impedance boundary conditions on the top and bottom surfaces of the domain. They appear green in the image. The left and right sides of the dielectric are terminated with open boundary conditions, shown in yellow in the image. The stripline is sourced by applying a wave port to the end of the trace.
The recommended port configuration for the stripline is the wave port on the end of the trace, as demonstrated.
Next, we show the real part of the electric field on the port plane.
These fields are appropriate for the fundamental TEM mode of the stripline.
The wave port is preferable here because it is inherently free of parasitics. That said, you can also set up a stripline with a lumped port if that's more appropriate to your system. To use a lumped port, define two lumped port patches that connect the trace to each ground plane, as follows.
By using two patches with your lumped port, you ensure that both ground planes carry return current.