Where is the welcome email generated in postfix?

Question

postfix

The bottom line is, when I register an account for a user in postfixadmin, he gets a welcome letter, I want to add corp information to this letter, tell someone where it is generated or compiled

Answer 1

Found the file config.inc.php
The line $conf['welcome_text']
It remains to understand how to push the html file there instead of the text