First, if you don’t know how to install custom code in SmartThings, take a look at this overview:
Then as far as creating an individual virtual presence device, there are several of them. The following is a nice one because it has the switch option, which means you can use anything in SmartThings that can flip a switch to change it from “home” to “away.”
There are many others that different community members have created that take somewhat different approaches. Some use Tasker and SharpTools with an android phone. Some connect to Wi-Fi. Some use a third-party service. So it depends in part whether you want a permanent device that will change Automatically, or whether you just want a quick switch that you can use as suggested in this thread, to substitute for a physical device which is acting up, or that you want to use for testing.