The loss of a via polygon is determined by the metal type it uses. When you draw a via polygon, its metal type is set to the default metal used for new vias or the metal type of a Via Technology Layer if it is being added to a Technology Layer. The default metal type is controlled in the Via metals tab of the Metal/Bricks page of the Circuit Settings dialog box. You access the Vias tab by selecting Circuit ⇒ Settings from the project editor main menu, then clicking on Metals/Bricks in the side bar menu of the Circuit Settings dialog box which appears. Once the Metals/Brick page is displayed, click on the Via metals tab. This metal type is used for new independent vias or for new Via Technology Layers.
You may also change the metal type of the via polygon or Via Technology Layer after it has been added to the circuit. You may use any via metal type defined in your circuit for a via polygon. For a complete discussion of how loss is modeled in via metal, see Via Metalization Loss.