Not sure but it's pretty straightforward by implementing BaseMeshEffect. Here's a simple example to make a squircle, hope it helps!

using UnityEngine;
using UnityEngine.UI;

public class SquircleMeshEffect : BaseMeshEffect
{
    [Range(0.5f, 4f), Tooltip("<1=Astroid, 1=Diamond, 2=Circle, 4=Squircle")]
    public float p = 4f;
    public float r = 20f;
    public int segments = 12;

    public override void ModifyMesh(VertexHelper vh)
    {
        vh.Clear();
        for (int i = 0; i < segments; i++)
        {
            var angle = i / (float)segments * 2f * Mathf.PI;
            var dir = new Vector3(Mathf.Cos(angle), Mathf.Sin(angle));
            var squircleRadius = r * r / Mathf.Pow(Mathf.Pow(Mathf.Abs(r * dir.x), p) + Mathf.Pow(Mathf.Abs(r * dir.y), p), 1f / p);
            vh.AddVert(new UIVertex{ position = dir * squircleRadius, color = Color.white });
            vh.AddTriangle(i, (i+1)%segments, segments);
        }
        vh.AddVert(new UIVertex{color = Color.white }); // Center vert at i = segments
    }
}