MRTK - Object Manipulation

I am working on object manipulation and I am trying to apply constraint manager at run time by script,

In this, I want to select manual constraint and add rotation axis constraint by script if anyone know how to do it If so please help me.

You need to refer to ConstraintManager Class (Microsoft.MixedReality.Toolkit.UI) | Microsoft Docs to understand this component, then you can refer to the example below.

using UnityEngine;
using Microsoft.MixedReality.Toolkit.UI;
using Microsoft.MixedReality.Toolkit.Utilities;

public class ConstraintTest : MonoBehaviour
{
    void Start()
    {
        RotationAxisConstraint rotationConstraint = this.gameObject.AddComponent<RotationAxisConstraint>();
        rotationConstraint.ConstraintOnRotation = AxisFlags.YAxis;

        ConstraintManager manager = GetComponent<ConstraintManager>();
        manager.AutoConstraintSelection = false;
        manager.AddConstraintToManualSelection(rotationConstraint);
    }
}