From what documentation they appear to provide, they're using features of the old plugin interface, not V4.
The main difference is that V4 allows plugins to read values directly, rather than needing them to be passed through the "Execute an External Plugin Function" action explicitly, and as such only uses the "Plugin Context" field.
This plugin, on the other hand, uses the "Plugin Context" field, and the "Variables" fields below it.
The "Plugin Context" field is used to specify an instruction, which from what I can tell may be "SetValue", for what you're trying to do.
A VoiceAttack variable must also be set; in their documentation the only type they use for "SetValue" is a Decimal variable.
That variable should be named identically to the ingame/SPAD.neXt value you're trying to set or get, E.G.
SIMCONNECT!LANDING_LIGHTS_TOGGLE
(I noticed you used a semicolon, however that's not mentioned anywhere in their documentation)
The value of that decimal variable might be 0 for off, 1 for on.
Ideally you'd check the value (E.G. using GetValue) while cycling through the different states ingame manually (E.G. using the in-cockpit controls, or an ingame keyboard shortcut)
The correct instructions, variable names, and values, are, as mentioned, specific to the third-party SPAD.neXt plugin/application and/or the flight simulator application you're using.
It would be the task of SPAD.neXt support to assist you with their product