Buttons can only be rectangular?

[Supremative]

You could create a custom button with a User Widget and check its alpha.

Follow these steps:

1. Create User Widget (WBP_CustomShapeButton) and add Image:
   

   

2. Add variables and Event Dispatchers like here:
   

   
   
   ButtonImage - Texture2D
   Normal, Hovered, Pressed - SlateBrush (Instance Editable)
   ButtonRenderTarget - CanvasRenderTarget2D
   IsMouseOver - Bool

3. Create Material that you draw to Render Target later:
   

   

4. Add Pre-Construct Event to Event Graph.
   

   
   
   
   

5. In the WBP_CustomShapeButton, create a Macro that will check alpha:
   

   

6. Override mouse functions OnMouseButtonDown, OnMouseButtonUp, OnMouseMove:
   

   
   
   
   
   
   
   

7. Override OnMouseLeave event:
   

   

Your custom shape button is completed.
Then apply it:

8. Add the button to your desired widget and fill the style:
   
   

Events:

Download Example Project (UE5)

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