RE: How to set Dynamics 365 Portal password to expire?
Not elegant however you could invoke this based on certain conditions i.e. time since last login.
Forgot password or password reset
Returning visitors who require a password reset (and have previously specified an email address on their user profile) can request a password reset token to be sent to their email account. A reset token allows its owner to choose a new password. Alternatively, the token can be abandoned, leaving the user’s original password unmodified.
Related site settings:
Authentication/Registration/ResetPasswordEnabled
Authentication/Registration/ResetPasswordRequiresConfirmedEmail
Related process: Send a password reset to a contact
Customize the email in the workflow as necessary.
Submit the email to invoke the process.
The visitor is prompted to check email.
The visitor receives the password reset email with instructions.
The visitor returns to the reset form.
The password reset is complete.