There are several ways to do this.
A hello home action is the first way.
Once you have one set up, you can then either use it from inside the ST mobile app (a bit tedious), or you can set up a widget to trigger it (fast and easy). ST widgets are now available for both Android and iOS, I’m not sure about Windows phone.
Alternatively, and good for any phone os, you can associate the hello home action (or your own custom smartapp) with a virtual switch, then use the IFTTT DO button app to flip that switch.
Also, you can make the virtual switch a tile in the free dashboard app, SmartTiles. Very popular, officially supported for installs, and very nice. You can have multiple versions, including one with just a few tiles for fast action:
So basically if you’re writing a custom smartapp, associate it with a virtual switch, then flip that switch directly in a SmartTiles tile or in the official ST mobile app on the Things screen, or wrap it in a Hello Home Action and trigger that as a widget or an IFTTT Do button.
It’s indirect, but once you have it set up, easy to use.
You will find virtual switches the key to solving a lot of use cases in ST. SmartApps and Hello Home Actions can subscribe to the events of a virtual switch, and the virtual switch fits both the official UI and most of the integrations with outside services.
This might be of interest as well: