Your port (a sheet object) must touch two different conducting objects on two opposite edges of the port object. Then one (and only one) of the conducting objects must be declared as the terminal reference. If the two sheet edges touch the same conducting object, then it is better to choose the modal solution type and draw an integration line. Alternatively, you can split the conducting object into two.

This is the simplest form where you have one terminal per port. However, it is possible to have more terminals, i.e. conductors which are not reference. This would be equivalent to different modes in the modal solution type.