Seems like you don’t do that. Not with UBTTask at least. They were designed to only be executed internally by the behavior tree framework.
So if in your UBTTask you need to use the functionality of some other UBTTask, the only way would be to subclass from it - but this is very, very bad from design perspective, as this kind of thing needs composition, not inheritance.
But, in the case of MoveTo task - it actually uses AITask, which is more low level task that allowes for composition. It is very clumbersome to use it directly, but luckily our AIController handles that for us.
My solution was to use AIController::MoveTo() and AIController::StopMovement() inside my task.