Get position on spline relative to world position

[Mos Def]

Hello

I am trying to Get position on spline or TF that is relative to world position. here in image the spline is 2D and its positioned at (0,0) and i have gizmo point (green one) that its position is (5,8,-4).

I need to get the position or TF on spline for a point that is also have same x as gizmo point (5).

Thanks