As of late July 2019, SmartThings Has added support for the five button IKEA handheld remote. This is a zigbee battery powered device available in both the US and the UK. All five buttons can be programmed for multiple options. Priced at around $15.99 in the US, £15 in the UK.
You have to use the new V3 app to program the device, but you don’t need any custom code for it. 
Note that although the product description will say it needs the IKEA Gateway, you can use the smartthings hub instead. And if you do use the smartthings hub, you can control any device that works with the hub, not just zigbee devices.
US
UK
https://www.ikea.com/gb/en/p/tradfri-remote-control-30338849/
Community discussion thread:
Ikea Tradfri Wireless control outlet kit and button now working with SmartThings (July 2019)
And @Automated_House has posted a video of the setup and the device working in post 19 of that thread.
Ikea Tradfri Wireless control outlet kit and button now working with SmartThings (July 2019)