Check out a guide to scripted extensions in the newest version by Luciano Iurino.

Unreal Engine 4.20 introduced a number of new features and tools. One of the coolest abilities here is the power to extend the Editor through Blueprints/Python scripting. The keys here are Actor and Asset Editor extensions. Wish to know more? Check out a guide to scripted extensions in the newest version by Luciano Iurino. 

A short piece to get you interested:

Scripted Extensions via Blueprint scripting need to be done via Blutility. If you don’t know what it is, Blutility is just a special type of Blueprint that helps create custom Editor-callable functions and events.

In order to use Blutility, you need to enable it in the Editor Preferences:

[Optional] Some of the tools exposed for the Editor scripting will only appear if you enable the Editor Scripting Utilities in the Plugin section:

First we need to create a new Blutility asset:

When asked for the parent class, just select ActorActionUtility if you want to extend the Editor to handle Actors in the scene:

Once created, if you want to open it for editing, don’t just double-click on it: it would just open an empty window. Instead, right-click on it in the Content Browser and click on Edit Blueprint:

Luciano Iurino 

Make sure to study the full guide here.