Change "power-button-action" comand for "hibernate" option in GNOME 3.18

This functionality was intentionally removed in commit 50564cde49ca2 (for bug 753713). After some discussion, it's returned in gnome-settings-daemon 3.20.

Unfortunately, 3.20 is not in the 16.04 ubuntu repos (however you can see that there is a 3.20 release for Yakkety 16.10).

gnome3-staging ppa contains an appropriate package (3.20.1-2ubuntu1~ubuntu16.04.1), but I believe that requires updating your entire gnome installation to the staging stream (which is like a beta).

I don't know if you can grab a deb of the right package and install that somehow.

This question has alternative instructions for how to modify the code and compile a working version.

I'm sorry there isn't a better answer for this and that these disruptive changes were introduced in a LTS release. You can vote on this bug to get 3.20 released in Xenial.