How to add a Lua DSP script?

Exactly what it says in the title – how do I add a Lua DSP script? The manual says to use the “New Lua Proc…” option in the mixer strip context menu, but I’m not seeing any such button.

I’m on Ardour 5.9, official build for OSX.

Just as (part) of the manual says, place the scripts in the $HOME/Library/Preferences/Ardour5/scripts folder. There doesn’t seem to be a “New Lua Proc…” button in any version of Ardour I have.

For Ardour6, user-scripts it would be $HOME/.config/ardour6/scripts/ and the script has to have a .lua file extension.