STEP 1 - CONNECT KEYBOARD AND MAKE SURE IT WORKS

• Connect any USB keyboard via an USB OTG cable or a bluetooth keyboard via USB
• Enter any app that has a text field (like Google Search for example) and press any key that would write some text, like the X key for example

If you can see that the keyboard works, you're ready to use it in Tasker with AutoInput

STEP 2 - CREATE PROFILE WITH "AUTOINPUT KEY" CONDITION

• Create a new Tasker profile
• Select the Event->plugin->AutoInput Key condition
• Select the X key
• Under Key Action select "Key Down"

This will make this profile trigger only when you press the X key down, and not when you release it. If you don't select this option, this condition will trigger twice for each key press (once for key down and another time for key up)

STEP 3 - OPEN XBOX ONE SMARTGLASS APP

• Add an "App" action and select the "Xbox One SmartGlass Beta" app
• Back out of Tasker to save everything

Now whenever you press the X key on your keyboard the app will launch, no matter where you are in the Android UI.
For more information, check out the AutoInput category page.