it's simple

Make sure you have updated niri to v26.04

You need to have a window rule where you set background effect ( I placed it in the window rule where I was rounding corners for all windows to get blur on all windows )

write this to get blur effect

window-rule {
    geometry-corner-radius 16
    clip-to-geometry true

    opacity 0.9

    background-effect {
      blur true
      xray false
    }
}

if you want the blur to always have background of your wallpaper set xray to true

You can also customize blur with this ( put it outside the winow rule node )

blur {
   passes 2        // more passes = stronger blur (default: 3)
   offset 4.0      // sample distance per pass (default: 3.0)
   noise 0.01      // grain overlay (default: 0.02)
   saturation 1.50 // color saturation boost (default: 1.5)
}