When you enable RPC (without a password), it creates the file automatically. -snip-
In the same directory, you will see the .cookie file if you already enabled RPC and restarted Core.
I tried removing the password on RPC and restarted Bitcoin Core. I saw the .cookie file created automatically in that folder.
The .cookie file disappears when Bitcoin Core is closed. It's like a temporary file.

Get that same directory and use it on the "Directory" field on bwt.
I have changed the directory and get the following error message when running Electrum (bwt) with Bitcoin Core at the same time.

1. Edit
2. Change RPC url (on Electrum) to use default RPC port