This isn't just unnecessary, but it'll makes the point of seed phrases meaningless. Why would you add a passphrase you cannot remember in another thing you cannot remember, and that's why you've written down instead, when it provides you no more essential security?

There's not even a reason to use bitaddress.org above iancoleman. Just generate twelve words and write them down. It is also not recommended to generate random numbers from javascript. Read Concern about RNG.