I tried it and it works. I used Ukraine as the country.
I am assuming anyone who wants to tank off the IL-78M will need to make this change. Or, save yours as backup and drop the attached in place of it.
Go to Scripts\Database\planes in your game folder.
Example: D:\Program Files\Eagle Dynamics\DCS World OpenBeta\Scripts\Database\planes
open il-78M.lua with notepad ++ or notepad
find "CanopyGeometry = makeAirplaneCanopyGeometry(LOOK_AVERAGE, LOOK_BAD, LOOK_BAD),"
below above line add
TACAN = true,
It will now look like this:
CanopyGeometry = makeAirplaneCanopyGeometry(LOOK_AVERAGE, LOOK_BAD, LOOK_BAD),
TACAN = true,
mapclasskey = "P0091000064",