How to connect external worker?

After fresh installation, I go into `Administration`->`Workers` and create a new worker.

- I select `Worker (a worker running on another host)`
- basename: foo
- password: secret
- count: 2
- I see two workers created but not connected (foo_1 and foo_2)
- I go into a server shell and create and start a worker

```bash
buildbot-worker create-worker foo_1 my-buildbot-travis-url foo_1 secret
buildbot-worker start foo_1
```

... and get:

```
2018-04-10 22:04:35+0200 [HangCheckProtocol,client] While trying to connect:
Traceback from remote host -- exceptions.RuntimeError: rejecting duplicate worker
```

So, how do I connect an external worker to buildbot-travis?