I've moved your question to the InDesign EPUB forum.
There is no such feature in EPUB. It's probably pretty dangerous opening up an app with a button. That would open you to all kinds of potential mischief.
My question was a little stupid!
What I want is links to social networks. For instance, a "Twitter" button can launch my twitter account on Safari [ie iPad, by default]. There's a little button on the Safari page that allows me to jump on the Twitter native app. Of course, this app. must be installed on the iPad. So the issue is better as it by default: the button in the ePub file simply launches Safari!
Thanks for your comment!