where to put premiere script, then where can i find it in premiere UI?

Theres no easy way to just load in a Premiere script as a menu item, like you might in After Effects.

However, there is a handy extension called JSX Launcher that allows you to load in jsx files you have prepared in Extendscript Toolkit, and then run them. No need to put them in a specific location, you can target whatever folder you would like.