You can always power off the screen with a 'top-swipe-down', then tap the padlock. In case you have the 'Quick app closing' option activated in >settings>gestures, you have to be in the app overview or the notification screen for this gesture.

As far as I remember there are only two options set by default for the power button. Short press is 'screen on/off', long press is 'shutdown'

There is an app called powermenu2 on http://openrepos.net that would allow you to configure the power button (that you don't have ). Some of us use it to restart the lipstick window manager in case we have a 'screen-on-freeze'. It looks like you can overcome this issue with some three finger tap on the screen if I remember well.

The openrepos software center app can be downloaded here https://openrepos.net/content/basil/...use-sailfishos. There are some cool apps available there.