You don't really want to do that.
Your applications should go in the Applications folder at the root level of your internal hard drive (on which the OS is installed).
If you try keeping your applications elsewhere, problems may arise "down the line" when you try to update those apps. The updaters may not be able to find them, or may have problems when doing the update.
Having said that, you CAN keep some applications elsewhere.
I wouldn't try this with Apple's apps -- only with 3rd party apps.
It's a better choice to store large aggregations of files (such as your music library, your photos, your movies) on the external drive, then to tell your apps to look for them "externally".