@Dhanlaxmi Jayapal (Tata Consultancy Services Limi) For your requirement, you can use "Send an HTTP request to Azure DevOps" action and use Comment Method Azure DevOps API.
You can use a html in the text of comment to refer to mention the user. Use a hyperlink and the data-vss-mention with the specific userid of the devops user you want to mention. To get userId, you can use "Get User" Action of Microsoft Entra Connector.
Below is an example of that. Replace ad96260c-ea80-6eeb-93b0-c942399631d0 by the correct userid.
{"text": "<div><a href=\"#\" data-vss-mention=\"version:2.0,ad96260c-ea80-6eeb-93b0-c942399631d0\">@John Doe</a> Test mentioning</div>"}
References documents :