From Xojo Documentation

Read-Only Property (As Integer )
IntegerValue = aControl.MouseY

Supported for all project types and targets.

The Y coordinate of the mouse (points). Measured from the top-left corner of the control.

Sample Code

This code is in the MouseDown event of a TextField and displays the Y-coordinate at the point of the MouseDown event.

Me.Value = Str(Me.MouseY)