Use the Springtrap pill and get to Stage 3. You'll see a menu that lets you choose between Stage 3 and Shadowtrap. If that doesn't work, then your game is scuffed

In Multiplayer Right?

how to get shadowtrap

The Easiest Way to get ShadowTrap is what Sonya Said ; Use the Springtrap pill and get to Stage 3. You'll see a menu that lets you choose between Stage 3 and Shadowtrap. If that doesn't work, then your game is scuffed (In Multiplayer)

soo how do i do that?

Go to a Multiplayer Server
(You don't have to have another person)
When You change Modes you have a menu that shows
Its easy, press on the Mode you want (there is Shadowtrap)
and you'll become that Mode, also the Animation is really cool for ShadowTrap

this is assuming you've unlocked him; which i can only assume the other people asking here have not
so to be clear: the menu will not show up until you've unlocked shadowtrap via random chance (very slight chance to become shadowtrap when transitioning to stage 3 as springtrap)