Hi @Lorraine Ogilby ,
Welcome to our forum!
Do you mean that you have added your boss's mailbox under Shared with me folder in New Outlook and you tried to reply to the email in his inbox, you found that a new window was opened, right?
If so, based on my research, i got the same test result as yours and didn't find any option/setting to change this behavior. I'm afraid that it's by design. If you truly need the function, it's suggested that you could turn off New Outlook button to switch to Legacy Outlook.
Please kindly understand that many functions are not designed at present. At present, what I can do for you is to submit feedback for your request and I have post here, and you could vote for it. If you have any suggestions or ideas, It's suggested that you could post here. Hope Microsoft will notice this in the future. Thank you for your understanding and support!
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