In Mods\aircraft\L-39C\Cockpit\clickabledata.lua add the following row in the "if not ASP_disabled then" block that starts at row 543 (Atleast in latest openbeta). The last four numbers are not correct (just copied from the mirror depression) but seems to work anyway.
elements["pnt_107"] = default_axis (_("ASP-3NMU Gunsight Target Distance") ,devices.ASP_3NM, device_commands.Button_2 , 107, 0.0, 0.1 , true, false, false, {135,45}, {135,10})
Edit: So it looks like this on my machine after the edit with version 2.9.2.49940