I want to create an outlook add-in for web, wherein I want a custom dialog box to open on click of Send button by the user?

Hi @Kannanm_1740 

Thank you for posting your question in the Microsoft Q&A forum. 

Regarding your issue, from my understanding new Outlook and Outlook on the Web (OWA) do not support COM add-ins but continue to work in classic Outlook for Windows so what worked for your classic Outlook might not work for new Outlook and Outlook on the Web. 

For more details, please refer to: Migrate from COM add-ins to web add-ins | Microsoft Learn 

Since I saw that you were trying to create a manifest file along with js and html and it didn't work, I highly recommend raising a ticket to Microsoft support. 

A technical support engineer can perform a remote session to investigate the situation, verify the backend configurations, and run any necessary synchronization tools to resolve the problem. If the issue requires further attention, they can escalate it to a specialized team for deeper analysis.

Here is what you should do next:

Submit a Service Request: The Global Admin should create a service request with Microsoft support to get advanced technical assistance and diagnostics. For detailed instructions on how to get support, please refer to Get support - Microsoft 365 admin.    

Apologies for redirecting you to the related development team support as the moderators in this community have limited resources to check the backend information, and to get the fast and better assistance we requested for it. 

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