You can add the SkeletonUtility
component just as you would add any component in Unity:
var skeletonUtility = component.gameObject.AddComponent<SkeletonUtility>();
To spawn the SkeletonUtilityBone
hierarchy, you can then use
skeletonUtility.SpawnHierarchy(SkeletonUtilityBone.Mode mode, bool pos, bool rot, bool sca)
You can have a look at the inspector code here:
spine-runtimes/SkeletonUtilityInspector.cs at 3.8