Maybe my English is bad, well it is,… But what are you asking?
You wrote. I made mailbox with a password reset. That mean including check the checkbox “Force password update at next login This user will only be able to login to Mail. App passwords remain useable.”, right?
And than you ask how to force user to change password on first login? You just did, or it is not working, or you are asking something different? I really do not follow sorry, please try to ask again in another sentence.