shaders onto ui elements

hello,

using the old canvas system, I was able to create any image element and assign it a material.
then, if I put it full screen, the shader could be used to create cool effects.
I mainly use this to make a blur in the background while UI is open.

how can I do this using UI Builder?
how to apply a shader? or create a blur in any way in the background???

ok, so the issue was to upgrade to unity 2021. now I can add renderTextures to the UI.