Hi @eciadmin_o365 ,
As I know, Outlook for Mac could not support the feature of changing the default email template when clicking the "new email" option yet. I also tried lots of tests in my environment and there is indeed no options for us to change that. I'm afraid that you could only manually open your email template to edit and send emails for now.
If you still have a demand to add this function to Outlook for Mac, we could create a thread in User voice, then other users will could vote on it for you, Microsoft may notice it and add it in the later version. For convenience, I have already created one here.
Hope this can be helpful.
If an Answer is helpful, please click "Accept Answer" and upvote it.
Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread.