I don't know how the scope companies do it, but I can assure you that you can't do it at home!
You don't just spray some in and screw on the "lid"!
It very likely done in some sort of vacuum or they are assembled in a sealed "box" that is filled with nitrogen(over simplification).
Why don't you call Leupold and ask them how they do it?
Generally, "O" rings are used for the sealer

Ole Cowboy