How to place a 3D object in the exact same space as 2D object on Screen?

I have a UI element that stretches appropriately to the screen. Inside that UI element I have this black square just for reference. I am trying to place a 3D object in world space on the same spot that this black square is. I am having trouble trying to do so.

I get the position of the UI element with this following code:

      Vector3[] corners = new Vector3[4];

      rect.GetWorldCorners(corners);

      Vector3 topLeft = Camera.main.ScreenToWorldPoint(corners[1]);

I get the top left corner of the UI object but when I do this, I am getting a negative value for these even though the position is on the screen and shouldnt be negative. As far as I am aware, GetWorldCorners starts in the bottom left and works clockwise so [1] should be top left.

Negative values coming from GetWorldCorners

Image of the UI element. Black square is just a placeholder to get the size that I am looking for with the 3D object