[IMcD23]

It's quite obvious. Just tried it myself [1].

You first select an identity to use to auto-fill. When doing that, it will tell you the data it will insert in the subtitle.

You can also click customize and choose what data to place into each field.

[1] - http://imgur.com/a/HL59q

[aioprisan]

It's obvious in Safari but not so in Chrome