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How would we create a UMG button using blueprint

How would we create a UMG button using blueprint in a way that it gets created when a player presses E on the already existing HUD rather than creating a a new HUD for a button. Add it to the existing HUD.

You can add a button into your existing BP which is set to “Collapsed” by default. In your player controller on “E” being pressed you can call a custom event which toggles/enables visibility of the button in your UI.

I would have said the classic create widget + add to viewport (the change being just you do it not at event beginplay or construct, but at the event you get when the player press E

your last phrase, mean you are just looking to add as a child perhaps?
if you have already a widget HUD to the viewport, and want to add a widget child to it, there is the “add child” node.