How to cast a simple ray on UI?

Hi,

I'm looking for a way to manually raycast on a canvas UI.
I need to feed it the point of origin, hitposition.
But I don't know how to do this.

I've been mooking at:
https://developer.oculus.com/reference/unity/1.42/class_o_v_r_raycaster/

But I can't make heads or tails of how to cast a simple ray on a UI.


Any help is greatly appreciated.
Jesper