Hey sentomo,
Welcome to the Magic Leap Developer Forums!
In Unreal, the bumper button on the controller is named "Shoulder"
To set this up, please navigate to Content/VRTemplate/Input/IMC_Weapon_Right to begin editing the controller mappings.
Under Mappings->IA_Shoot_Right, you should see a drop down that allows you to select a button to map this action to.
From that dropdown, you will want to select Magic Leap Controller->Magic Leap (R) Shoulder
Please let me know if this works for you!