I have a *.js file which can be used to type bengali in text area how to do that

I want that my user can write story in Bengali. I have a custom javascript (phoneticunicode.js) I have to declare

befor the and I have to call it later by passing the ID of the text area in order to use my custom script to write bengli in text area. I can do it in normal HTML file but how can I impleament it in Drupal?