To Drop a Perpendicular Line From a 3D Point to the XY Plane

Click Drafting tab > Draw panel > Line.
Use an object snap to specify a point on an object that is not on the XY plane of the UCS.
This location defines the first point of the line.
Enter .xy and then enter @ at the Of prompt.
This operation extracts the X and Y coordinate values from the first point.
Enter 0 to specify the Z value.
The X and Y values extracted from the first point are combined with a new Z value (0) to finish the definition of the second point.
Press Enter to end the command.