We wanted to include a link in one of our macros that folks could click and get directly into a chat session. I contacted Zendesk support an they got the answer so I thought I'd share it here. Big thanks to Zsa T. | Technical Support Engineer - APAC, who tracked down this answer. Works perfectly.
You can use this URL to open a new page for the chat window instead of opening on the main page.
To get the account key, kindly click on the avatar and choose "Check connection".