I am afraid that you will have to wait until later in the season to use any insecticide for Magnolia Scale control. They are safe under a protective wax coating at this time of year and there are no insecticides that will affect them. In the fall (late August), horticultural oil (summer oil) can be used to control the crawler stage of these scale insects. In the meantime, pruning off the most heavily infected branches can help the tree and reduce the amount of scales that you will have next season.If you choose to use an insecticide, be sure to follow all label directions to ensure the product's effectiveness and safety. Other control options can be found in this MSU Extension article: https://www.canr.msu.edu/news/check_magnolias_for_magnolia_scale_now

The scale infestations look bad and create a mess, but they are not as harmful to the tree as you might think. Usually, beneficial insects like predators and parasitoids help bring the infestation back down to acceptable levels within a year or two. You can use plain water sprays with a garden hose to wash the honeydew off of the leaves. This will help the plant look better and reduce the numbers of ants and wasps that are attracted to the honeydew.