Get UI ScreenPosition & WorldPosition in separate Camera & Canvas

UIScreenPos.cs

// Scene setup:
// CanvasCamera (canvasCam) with ScreenSpace-Camera UI canvas & elements (in different position from main camera)
// MainCamera (mainCam)
// UI image (obj) inside the CanvasCamera canvas

var originalScreenPos = canvasCam.WorldToScreenPoint(obj.position);
var worldPosInMainCamera = mainCam.ScreenToWorldPoint(p1);
var rayDirection = (worldPosInMainCamera-mainCam.transform.position);
Debug.DrawRay(mainCam.transform.position, rayDirection * 999, Color.red);

RaycastHit hit;
if (Physics.Raycast(mainCam.transform.position, rayDirection, out hit, Mathf.Infinity, layerMask))
{
    Debug.Log("Hits object:" + hit.transform.name);
}