ソースコード

using UnityEngine;
using UnityEngine.Splines;

public class Example : MonoBehaviour
{
    [SerializeField] private SplineContainer m_splineContainer;

    private void Awake()
    {
        var spline = m_splineContainer.Spline;

        spline.Add( new( Vector3.left  ) );
        spline.Add( new( Vector3.zero  ) );
        spline.Add( new( Vector3.right ) );
    }
}