can anyone help me fetch only Sign in events from the api
We can't help with Postman itself as that's not made by Dropbox, but on the Dropbox side of things it sounds like you would want to use the /2/team_log/get_events and /2/team_log/get_events/continue endpoints to retrieve event information like this. You can set event_type to login_success to retrieve successful login events.
You can also use the API v2 Explorer to prototype and test calls like this. You can configure it with your desired parameters, and click "Show Code" to see how the request would look in a few different formats.
