Certified GKH Gardening Expert
It is important that, although they are interesting a largely not harmful, when a tree is not getting proper care or is lacking something, then the parasitic plant will steal nutrients from the tree.
In short, they will help facilitate the death of a tree in the right circumstances.
This article will help you to grow mistletoe, both indoors, and out: https://www.gardeningknowhow.com/ornamental/flowers/mistletoe/how-to-grow-mistletoe-plants.htm
This article will offer information on mistletoe and other parasitic plants: https://www.gardeningknowhow.com/plant-problems/environmental/hemiparasitic-plants.htm