How can I input text that varies into an email macro?

[Airy]

@pauldev

However, that doesn't work inside the text input box. Is that right?

Correct. That can't work.

I'll see if I can find the thread that explains why, and post it here.

User Prompt and Text Expansion Not Working Together

You can't use typed string triggers inside prompt for user input text fields. See the wiki here action:Prompt for User Input [Keyboard Maestro Wiki]