Check out post #4 on this thread, https://forums.unrealengine.com/development-discussion/vr-ar-development/1444157-handheld-ar-world-origin. His blueprint network does a line trace from the center of the screen to an underlying AR plane, moves the world alignment to this hit point, and rotates the world alignment about its z-axis so as to align its x- and y-axes to the camera.