How to check if Trigger button is pressed for Windows Mixed Reality ?,How to detect buttons on Windows Mixed Reality controllers

Right now I am developing a game that the player need to select an object (point the pointer at the game object and press the "Trigger Select Button" on controllers).

I spent a lot of time googling and still couldn't figure out how to do t,

I tired if (Input.GetButtonDown("xxxxxxx")) { Debug.Log("xxxxx is held down"); }

But I don't know what the Button name is the WMR trigger button.

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