In Phabricator, how to assign a parent to an existing task, change it from task to sub-task

It's not clear in the Phabricator interface or documentation how I can assign a parent to an existing task. In trac, I just add the "parent" ticket number, and the task automatically becomes a subtask of the new parent.

How do I accomplish the same in Phabricator so that an existing task becomes a child of another task.