The code is supposed to fill out a default Subject to a message.
This is the message I got from BP support, when I raised the question a while back.
Since it is just 2 lines of code, My suggestion will be to put it in one of the js file provided by theme. Look for a file named buddypress.js and you can put the code inside the dom ready section there. If you are using a custom theme, you may ask there(or check their documents) to see if there is any custom js file supported.