I have never written a screenplay, so take this with a grain of salt. But from the few screenplays that I have read, I can tell that they often have quite a lot of detail. So if you want to have symbolism in your screenplay, just add descriptions as necessary. You can also add suggestions for the camera work, but I would not go into too much detail here, since the actual camera work is not your department. But if a scene depends on a visual detail or on a camera movement, just write it into the description.
ALICE wanders through a hallway made of strangely arranged PILLARS. The camera follows her gaze and slowly pans the room. It seems disjointed and confusing. But then, the camera turns around, revealing a perspective in which the pillars form a perfect GIGANTIC TRIANGLE. The triangle symbolizes Christian dogma that has haunted Alice all her life, and is an important reoccurring symbol.
So you establish that the triangle is a symbol you are consciously using, and you describe how this scene depends on a certain camera trick to make it work. This belongs in the screenplay because it is just as necessary as, for example, details about the location a scene is set in.
Here is an example from a real movie screenplay - it's the early version of Dark City by Alex Proyas, the first movie with a lot of symbolism that came to my mind.
Bumstead shakes his head slowly, turns away from the board.
Picks up a cup of hot tea, pours milk into it from a small jar.
TIGHT ON THE TEA CUP - Cream SWIRLS into a rapidly dissolving
spiral. Bumstead looks up. Puts down the cup hurriedly, turns
back to the map.
With a thick pen he traces a line between each point marking
the location of the victims. He steps back to examine his
A SPIRAL - moving outwards. Beyond the last victim it
becomes a dotted line, following the same trajectory but with a
big question mark beside it.
While it is not explicitly stated, you can clearly see that the spiral is an important symbol. The camera work is also described in detail (actually more detailed than I would have thought).