MethodExtensions.cs

using Cinemachine;
using UnityEngine;

public static class MethodExtensions
{
    public static Vector3 WorldToViewportPoint(this CinemachineVirtualCameraBase vcam, Vector3 position)
    {
        // CyRaid https://forum.unity.com/threads/reproducing-cameras-worldtocameramatrix.365645/
        Matrix4x4 worldToCameraMatrix = Matrix4x4.Inverse(Matrix4x4.TRS(vcam.State.FinalPosition, vcam.State.FinalOrientation, new Vector3(1, 1, -1)));
        Matrix4x4 projectionMatrix = Matrix4x4.Perspective(vcam.State.Lens.FieldOfView, vcam.State.Lens.Aspect, vcam.State.Lens.NearClipPlane, vcam.State.Lens.FarClipPlane);

        // Thanks to bgolus for being the hero of the Unity community
        // https://forum.unity.com/threads/camera-worldtoviewportpoint-math.644383/
        Vector4 worldPos = new Vector4(position.x, position.y, position.z, 1.0f);
        Vector4 viewPos = worldToCameraMatrix * worldPos;
        Vector4 projPos = projectionMatrix * viewPos;
        Vector3 ndcPos = new Vector3(projPos.x / projPos.w, projPos.y / projPos.w, projPos.z / projPos.w);
        return new Vector3(ndcPos.x * 0.5f + 0.5f, ndcPos.y * 0.5f + 0.5f, -viewPos.z);
    }
}