Step 1:
Enter FNAF 3 Night 1, nothing will harm you here.
I read there used to be the highest chance to get jumpscared at the prize section here,
so we will just use this night to "theoretically" increase our chances.
Unfortunately, the 100% chance of getting jumpscared here isn't a thing anymore, but don't worry!

Step 2:
When arrived in the office, skip the night instantly to obtain your prize.
Press and Hold ALT + SHIFT + U for a few seconds until the screen fades black.

Step 3:
You now managed to win the game in no time. Collect your prize.
If you get jumpscared by plushtrap now, you did it!
Remember: It's rare, though. Only 1/3 of the players encountered plushtrap.

Didn't get the jumpscare prize?
Step 4:
Repeat the process.
Just enter FNAF 3 Night 1 again and skip it with ALT + SHIFT + U.
One game only takes a few seconds. Repeat this until you get what you're looking for.

This also works for already completed games!
You completed the prize counter if every prize you get is a white piece of cake.