I want to reserve an app name that I intend to build out over the next 90 days, how do I do this in apple\'s web developer portal?
You can add the application as suggested above. And about app name reservation here is my answer.
After creating your app and it is in the state Prepare For Upload or Waiting For Upload, you have 180 days (6 months) from your creation date in iTunes Connect to deliver a binary to Apple. If you do not deliver a binary before the 180-day deadline, your app is deleted from iTunes Connect. As a result of this deletion, your app name can be used by another developer and you cannot reuse the app name, SKU or bundle ID.
I got this information from apple documentation and the link is
http://developer.apple.com/library/ios/#documentation/LanguagesUtilities/Conceptual/iTunesConnect_Guide/8_AddingNewApps/AddingNewApps.html#//apple_ref/doc/uid/TP40011225-CH13-SW1
Search for "App Name Expiry" and it leads you to the required page.
I hope this helps all of us.