After clicking "Open an existing Android Studio project", a dialog pops up that shows the directory tree, and buttons on top. The third button is "Project Directory" (Ctrl+2), which starts grayed-out, and presumably it is there so you can set a designated directory so you can open existing projects faster. How to set it?
Searching "Project Directory" in the IDE settings gives several irrelevant results. Pressing a "?" button on the dialog leads to a 404 page. Googling also gives no results. Does somebody here know enable this feature?
(Android Studio 3.0.1 on Ubuntu 16.04.)