In most of the MSSR examples, the “Grab” event checks if the actor has a tag, usually “pickup”. Here, in the VR example, the check is in the “GetActorsNearHand” method (see pic). You can check one of the cubes, look for Actor Tags array in the properties. So you have to add a tag like that to your actor.
You can also change the code of the Grab to check some other criteria, like just grab anything which is a physics actor. The original example used an Interface PickupActorInterface to see what you can pick, but I don’t like this approach, because it forces you to use a custom actors. A tag can be added to any actor, including basic static mesh actors.
And to “snap” you need to have sockets defined on those meshes, but I suppose you know that already. If not, check Overview level in the example project and the tutorial video.