Not as simple as other toggles. The game resets pressed keybinds when opening a screen, and setting the attack key to be pressed when the screen changes, as is done with the hold right click toggle, doesn't work. Attempting to mess with the inner workings of the game has issues that are too complicated for me to solve right now, so the feature is disabled for now.

This can be closed because minecraft offers this

Click and then press f3-t to get auto hold left click