https://stackoverflow.com/questions/66002667/pinvoking-user32-dll-setwindowcompositionattribute-via-powershell

NativeMethods.cs

using System;
using System.Runtime.InteropServices;

// https://stackoverflow.com/questions/66002667/pinvoking-user32-dll-setwindowcompositionattribute-via-powershell
public static class NativeMethods
{
    [DllImport("user32.dll")]
    public static extern bool SetForegroundWindow(IntPtr hWnd);

    [DllImport("user32.dll")]
    public static extern bool ShowWindow(IntPtr hWnd, int nCmdShow);

    // See http://pinvoke.net/default.aspx/Enums/DwmSetWindowAttribute.html

    public enum DWMWINDOWATTRIBUTE : uint
    {
        NCRenderingEnabled = 1,
        NCRenderingPolicy,
        TransitionsForceDisabled,
        AllowNCPaint,
        CaptionButtonBounds,
        NonClientRtlLayout,
        ForceIconicRepresentation,
        Flip3DPolicy,
        ExtendedFrameBounds,
        HasIconicBitmap,
        DisallowPeek,
        ExcludedFromPeek,
        Cloak,
        Cloaked,
        FreezeRepresentation
    }

    [DllImport("dwmapi.dll", PreserveSig = true)]
    public static extern int DwmSetWindowAttribute(IntPtr hwnd, DWMWINDOWATTRIBUTE attr, ref int attrValue, int attrSize);

    // Grabbed from https://gist.github.com/riverar/fd6525579d6bbafc6e48

    [DllImport("user32.dll")]
    private static extern int SetWindowCompositionAttribute(IntPtr hwnd, ref WindowCompositionAttributeData data);

    [StructLayout(LayoutKind.Sequential)]
    private struct WindowCompositionAttributeData
    {
        public WindowCompositionAttribute Attribute;
        public IntPtr Data;
        public int SizeOfData;
    }

    private enum WindowCompositionAttribute
    {
        // ...
        WCA_ACCENT_POLICY = 19
        // ...
    }

    private enum AccentState
    {
        ACCENT_DISABLED = 0,
        ACCENT_ENABLE_GRADIENT = 1,
        ACCENT_ENABLE_TRANSPARENTGRADIENT = 2,
        ACCENT_ENABLE_BLURBEHIND = 3,
        ACCENT_INVALID_STATE = 4
    }

    [StructLayout(LayoutKind.Sequential)]
    private struct AccentPolicy
    {
        public AccentState AccentState;
        public int AccentFlags;
        public int GradientColor;
        public int AnimationId;
    }

    // And here, expose some methods that will simplify the use of SetWindowCompositionAttribute from the outside
    // Adapted from https://www.kechuang.org/t/79675
    public static void EnableBlur(IntPtr HWnd, bool hasFrame = true)
    {
        var accent = new AccentPolicy
        {
            AccentState = AccentState.ACCENT_ENABLE_BLURBEHIND
        };

        if (hasFrame)
            accent.AccentFlags = 0x20 | 0x40 | 0x80 | 0x100;

        var accentStructSize = Marshal.SizeOf(accent);

        var accentPtr = Marshal.AllocHGlobal(accentStructSize);
        Marshal.StructureToPtr(accent, accentPtr, false);

        var data = new WindowCompositionAttributeData
        {
            Attribute = WindowCompositionAttribute.WCA_ACCENT_POLICY,
            SizeOfData = accentStructSize,
            Data = accentPtr
        };

        _ = SetWindowCompositionAttribute(HWnd, ref data);

        Marshal.FreeHGlobal(accentPtr);
    }
}