BETA feature: This is meant for testing and not recommended for production.
Before you are able to use BP_CharacterCustomization > Update Skeletal Merging, you have to follow this tutorial.
Since we submit content and not a full project to Unreal Marketplace, we cannot decide which plugins are activated in your project, thus you have to manually active the built-in Unreal Engine plugin "Skeletal Merging".
Open Unreal Engine > Edit > Plugins, search for the built-in plugin "Skeletal Merging" and activate it.

Restart Unreal Engine.
Open BP_CharacterCustomization > UpdateSkeletalMerging.

Navigate to the red comment and insert the nodes Merge Meshes and Make SkeletalMeshMergeParams as shown below.

Re-compile BP_CharacterCustomization.
Read BP_CharacterCustomization > Update Skeletal Merging to see how to use this feature.