SHADOWTRAP OBTAINMENT:
To obtain shadowtrap, you must hit the 1/2200 chance when pressing "R" on Stage 2. (Transforming into Stage 3). if you hit the chance, you will be met with a pop-up asking if you want to transform into Stage 3 or the Shadowtrap.

IMPORTANT NOTES*
if you wish to obtain shadowtrap and KEEP it, you must do it in multiplayer server (Opening an atleast 2 person server, you can still do it alone.) Once you unlock shadowtrap in MULTIPLAYER. All you have to do is press "R" in Stage 2 springtrap and it will always ask you if you want to do stage 3 or Shadowtrap.

Personal notes:
an easy way to get shadowtrap (legitimately) is having a key presser binded to "R" and leave it overnight. If you want to do it manually, Press "R" on stage 2 and re-morph into springtrap again before the transforming animation is finished, this will let you bypass the stage 3 cooldown.

You can use the Easy Animation tool to view Shadowtrap's animations and springtrap's shadowtrap transformation animation, it's not the same animation as transforming into S3. Have a great grinding sesh.