If "Solid123" is the name of the body, then write:
Dim obj = Snap.NX.NXObject.FindByName("Solid123")
obj.IsHighlighted = True
Internally, the code will still cycle through the part file looking for "Solid123", though. This is not very efficient, but, depending on what you're doing, you may not notice the inefficiency.
Be careful ... feature names and body names are two different things, so you have to think a bit about which one you're using.
There is usually no need for this "FindByName" stuff. Usually, you already have a program variable that refers to the object you want to highlight (because you created it, or the user selected it, for example). So, I'd recommend that you take another look at your code, and see if you already have the object you need.