Use for FinalIK

HandIkAxisSettingSolver.cs

using System.Collections;
using System.Collections.Generic;
using RootMotion;
using UnityEngine;

public class HandIkAxisSettingSolver : MonoBehaviour
{
    [SerializeField] private Transform foreArm;
    [SerializeField] private Transform hand;
    // Start is called before the first frame update
    void Awake()
    {
        if (foreArm != null && hand != null)
        {
            var wristToPalmAxis = GuessWristToPalmAxis(hand, foreArm);
            Debug.Log("WristToPalmAxis");
            Debug.Log (wristToPalmAxis);
        }

        {
            var palmToThumbAxis = GuessPalmToThumbAxis(hand, foreArm);
            Debug.Log("PalmToThumbAxis");
            Debug.Log (palmToThumbAxis);
        }
        
    }
    
    public static Vector3 GuessWristToPalmAxis(Transform hand, Transform forearm)
    {
        Vector3 toForearm = forearm.position - hand.position;
        Vector3 axis = AxisTools.ToVector3(AxisTools.GetAxisToDirection(hand, toForearm));
        if (Vector3.Dot(toForearm, hand.rotation * axis) > 0f) axis = -axis;
        return axis;
    }
    
    
    public static Vector3 GuessPalmToThumbAxis(Transform hand, Transform forearm)
    {
        if (hand.childCount == 0)
        {
            Debug.LogWarning("Hand " + hand.name + " does not have any fingers, VRIK can not guess the hand bone's orientation. Please assign 'Wrist To Palm Axis' and 'Palm To Thumb Axis' manually for both arms in VRIK settings.", hand);
            return Vector3.zero;
        }

        float closestSqrMag = Mathf.Infinity;
        int thumbIndex = 0;

        for (int i = 0; i < hand.childCount; i++)
        {
            float sqrMag = Vector3.SqrMagnitude(hand.GetChild(i).position - hand.position);
            if (sqrMag < closestSqrMag)
            {
                closestSqrMag = sqrMag;
                thumbIndex = i;
            }
        }

        Vector3 handNormal = Vector3.Cross(hand.position - forearm.position, hand.GetChild(thumbIndex).position - hand.position);
        Vector3 toThumb = Vector3.Cross(handNormal, hand.position - forearm.position);
        Vector3 axis = AxisTools.ToVector3(AxisTools.GetAxisToDirection(hand, toThumb));
        if (Vector3.Dot(toThumb, hand.rotation * axis) < 0f) axis = -axis;
        return axis;
    }
}