Hi @M. R. Chaturvedi
Welcome to our forum!
It looks like your question is more related to Outlook development, please understand the general problem that Outlook tabs are focused on the client, and don't know much about development, the following is just for reference.
As far as I know, the pin option in Outlook Task pane Add-Ins is typically controlled by the manifest file. However, there are certain scenarios where the pin option might appear even if it's not explicitly configured. For instance, in Outlook on the web and the new Outlook on Windows, the task pane of an add-in that implements the item multi-select feature is automatically pinned to the Outlook client.
More information, see: Implement a pinnable task pane in Outlook.
By the way, you can get more professional advice on issues related to developing add-ins under this Office Development tab. Thank you for your understanding and patience!
If the answer is helpful, please click "Accept Answer" and kindly upvote it. If you have extra questions about this answer, please click "Comment".
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.