Do you set in man in Minecraft 1.12 that you sneakt continuously?
As far as I know, not possible with Minecraft alone.
However, if you have a program on your computer that keeps pressing the key all the time, that shouldn't be a problem.
No only when you press the button or use plugins when you play on the console but there should be a setting where mam sneaks after pressing a button until you press again but no idea where
There are mods for that. Googlesneak