Hello! If you are using Spine PRO (which is required for being able to define IK Constraints in Spine Editor), then the animator/rigger needs to setup an IK Constraint.

Then at runtime, all you need to do is to control the IK Target bone position. The Spine-Unity runtime provides a method that allows you to set a bone position according to a given Unity world position:

using UnityEngine;
using Spine.Unity;

public class BoneControllerSample : MonoBehaviour {

   [SpineBone]
   public string boneName;
   Spine.Bone bone;
   public Vector3 targetPosition;

   void Start () {
      SkeletonAnimation skeletonAnimation = GetComponent<SkeletonAnimation>();
      this.bone = skeletonAnimation.Skeleton.FindBone(boneName);
      skeletonAnimation.UpdateLocal += SkeletonAnimation_UpdateLocal;
   }

   void SkeletonAnimation_UpdateLocal (ISkeletonAnimation animated) {
      var localPositon = transform.InverseTransformPoint(targetPosition);
      bone.SetPosition(localPositon);
   }

}

If you are using Spine Essential, you can't set up an IK Constraint. So you'd need to do the math yourself. If that's the case, having a simpler skeleton (like in the video you linked) will make things easier.

using UnityEngine;
using Spine.Unity;

public class BoneControllerSample : MonoBehaviour {

   [SpineBone]
   public string boneName;
   Spine.Bone bone;
   public Vector3 targetPosition;

   void Start () {
      SkeletonAnimation skeletonAnimation = GetComponent<SkeletonAnimation>();
      this.bone = skeletonAnimation.Skeleton.FindBone(boneName);
      skeletonAnimation.UpdateWorld += SkeletonAnimation_UpdateWorld;
   }

   void SkeletonAnimation_UpdateWorld (ISkeletonAnimation animated) {
      var bonePosition = bone.GetWorldPosition(this.transform);
      var direction = targetPosition - bonePosition;
      float rotation = DirectionToRotation(direction, this.transform);
      float parentRotation = bone.parent.WorldRotationX;
      bone.RotateWorld(rotation - parentRotation);
   }

   static float DirectionToRotation (Vector3 direction, Transform transform) {
      var localDirection = transform.InverseTransformDirection(direction);
      return Mathf.Atan2(localDirection.y, localDirection.x) * Mathf.Rad2Deg;
   }

}

It's a NullReferenceException.
You probably had the wrong bone name.

If the bone name is inspector-assigned, we recommend using the [SpineBone] attribute in code so the inspector shows a list of bones you can select from, instead of needing you to manually type the name correctly.

[SpineBone]
public string boneName;