For Gmail's SMTP server to accept a message From another email, it *has* to be configured on Gmail's web site first.
Without it, the different From address looks to Gmail like an attempt at identity forgery.
Once that's done, you should be able to add the identity in my app without configuring a separate outgoing server for it. It will just use the parent account's outgoing server (smtp.gmail.com, which should now accept the message).