Currently, you have to sign into Server via the app. I don't believe the functionality to open the app from a link is available.
Hey Wenhui --
+1 on Tracy's response. In order to do this, our iPad app would need a custom URL Scheme available for public use -- no such animal. If you think this would be useful - please add an "Idea" in our ideas forum requesting that Tableau create and add a Custom URL Scheme for our app - I personally think it would be a great thing!