You're looking for an L:Var that triggers the light, or an L:Var that indicates when the light is illuminated?
For the latter, there are instructions in the manual under the "Annunciator Lights" section that describe how to access L:Vars for most of the annunciator lights in my aircraft. In this case, it should be L:var_MD41Message_EM_readonly.
For the former, no such L:Var exists to trigger the light, since the lights all have complex behavior and I haven't been expecting users to drive them with their own logic, but if this is something you really wanted to do, you could add (L:MD41_MeggageLight, bool) or to BonanzaProfessional_Base_INT.XML in the block of code containing <NODE_ID>MD41Message_EM</NODE_ID>, directly before the line, </ON_CONDITION_CODE>.