Customizing user invitation messages on a per user basis

Fellow AWS devs,

I have a question about modifying the invitation emails sent out by Cognito after creating users with the adminCreateUser method.
For the email verification messages I managed to adjust the mails using a custom lambda method that is checking for event.triggerSource === "CustomMessage_ForgotPassword" . I set this lambda to be triggered under triggers in the Cognito console under Custom message entry.

When adminCreateUser is called for a new user, I do see the same lambda method being trigger, just this time with event.triggerSource === "CustomMessage_AdminCreateUser" .
But when I adjust the text with

   event.response.emailMessage = htmlBody;

, the email received is still the static text setup in the cognito console, not the modified one. Modifying the response.emailMessage for the verification mails works though.

Whats the trick getting user-invitation emails modified?

PS: the static email text in the cognito console is unfortunately not enough, as I have different clients using the same cognito pool and want to modify the emails on a per-client basis.

Edited by: Bersaelor on Apr 29, 2020 3:59 PM