How do I set up an automated welcome email using custom code?

I'm trying to create an automated welcome email when users sign up to a list. I have the email I'd like to send out already built as a custom code email, but when I select the option to "copy an existing email" none of the emails I've built show up. If I instead choose to "create a new email", the option to create the email with custom code isn't available:

Typically when creating an email, the option to use custom code is listed:

Is there any way to get around this and use custom code when working with automated emails?