From the Routers page, click on Create Router in the upper-right corner of the page. A window will pop up where the name of the router to be created should be specified as follows:

Enter the name of the router, and click on the blue Create Router button to complete the operation.

From the Routers page, click on the Set Gateway button found in the Actions column that corresponds to the router. A window will open that allows you to set the external gateway network:

To confirm the gateway network selection, click on the blue Set Gateway button.