What is my https://proxmail101.his.com password?

There is no special password to login to the Proxmox Mail Gateway:

Use one the links to the web interface located with the messages, or use the link located at the bottom of the Daily Spam Report you received to see more messages currently in your quarantine account.

For Zimbra-hosted customer domains and his.com accounts:

Your username and password on https://proxmail101.his.com are your mail mailbox email address and your email password.

Log in at https://proxmail101.his.com using your email address user@mydomain.com and the password that you use to pick up your mail.

Note: if you have your user@mydomain.com or user@hers.com email address mapped or forwarding to a mail.his.com mailbox, you MUST log in using the main his.com username (user123@his.com) and password. You will not be able to login using an alias, or with just the username (the left part of your email address)