You can't, sorry. Tiger strikes and below don't drop weapon engrams from the reward screen.
English
-
But you can get them in the actual strike which are still identified as Vanilla engrams.
-
Any engrams that drop from enemies are HoW engrams.
-
How do you know this?
-
They have different descriptions.
-
Happened to me. Got a HoW legendary from a blue engram that I picked up in a tiger strike
-
Vanilla engrams have a different description than HoW engrams.