Hi @Guyre, Mike
Welcome to our forum!
Based on your description, this seems to be the expected behavior for joining a Teams meeting directly from New Outlook. It allows you to quickly enter a Teams meeting even when you don't have a Teams client and doesn't support turning off this prompt at this time.
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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