概要

using UnityEditor;
using UnityEditor.Build;
using UnityEngine;

internal static class Example
{
    [MenuItem( "Tools/Example" )]
    private static void Run()
    {
        var icon = AssetDatabase.LoadAssetAtPath<Texture2D>( "" );

        PlayerSettings.SetIcons
        (
            buildTarget: NamedBuildTarget.Unknown,
            icons: new[] { icon },
            kind: IconKind.Any
        );

        AssetDatabase.SaveAssets();
    }
}

参考サイト様

https://discussions.unity.com/t/unity-2021-setting-the-default-icon-via-script/249169