I want to make a background blur UI that looks like below…
By the way, I know that the background blender widget is only square and round.
What should I do to make the same button below? Is there a way to use the background blur widget as a mask?
The Retained Box, most likely:
“Most likely” because I made a lot things with retainers but never a custom shaped blur. Curious. Do tell if that works!
It does not work 
unfortunately retainer boxes do not accept blurs and i need to have a custom shaped one so bad for my HUD
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I’d then look at:
https://www.fab.com/listings/69680f34-e5d2-44e6-b023-f054bbf629eb
And look into signed distance fields, you might be able to pull it off. See the docs of the abovementioned product and look up: “signed distance fields”. It’s generally for primitive shapes but perhaps you can get it to work by approximation.
And, lastly, there are new UMG features on the way:
This one looks extra promising (back buffer):
Perhaps something gives.