alt Hacker NewsIf by "input box" you mean "drop down list" and by "anything" you mean "any of my account's configured email addresses" then yes.. but FastMail doesn't just let you put anything in the From field - on the web or Android as far as what my subscription shows me.
It definitely does for me in both web and android. You can setup specific hardcoded aliases for routing purposes but I have a wildcard/catchall and I can put anything in the From when I compose or reply to an email.