There are two options now.
1. Have at least one premium stash tab set as public. In this case you don't have to do anything and your online status will be determined automatically.
2. If you don't have any premium/public stash tabs: Go here. Send any message. Wait for a reply. You'll get an "online link". You should then input it into a program such as Acquisition and your online status will be tracked while Acquisition is open.