In K2, it moved. After emails to support we found that it now hides near the top of the Agent Status page, between the “Select Columns” and “Filter” button.
When you click on this, you will actually open a new .asp page (IE recommened/required).
At the top of the page you can enter the Proxy Server information, including port and login information if needed (bullet #1). Then you can apply those setting to the system(s) (#2) as required.