I want to add a horizontal surface to a 3D Wall Plot at a certain value. The image represents three dunes, and the horizontal surface should mark the 5,7m border (so the value is 5,7 on Z)
A work around method is to change the position of the ZX Plane.
Double click the graph to open the Plot Details. Then Select Layer 1 in the Left Panel. Select Planes. For ZX select At Position = for the Position option in the dropdown menu and input 5.7. I realize this isn't exactly what you are asking for but it might be "good enough".
You can also edit the axis labels so that they are positioned in a different location on the graph.