For a zwave classic device, The easiest way is probably to use a minimote. 
These are available on both the US and UK frequency. ( you can also use it for zwave devices, it’s just that you probably won’t need to.)
A minimote has two completely separate functions. On an every day basis, it can serve as a handheld remote, and each of the four visible buttons can be assigned to anything that SmartThings can do. So it can turn on another device, change the mode, arm the smart home monitor feature, etc.
In addition, and this is what is relevant here, there are four buttons inside a slider compartment that can be used for administrative functions, including pairing New devices to your network. Very useful.
Just make sure you get the correct frequency for your region.
US:
http://www.zwaveproducts.com/shop/controllers/z-wave-remotes/z-wave-minimote
UK:
And here’s the community FAQ on the process: