Select a main source (e.g. RADIO, MEDIA).
Select the source (e.g. AM, Bluetooth).
In the source's normal view, press OK/MENU and select the FAV menu.
Alternatively long press on FAV button, until the menu is shown.
Select a function in the menu item to link to FAV.
When the source (e.g. AM, Bluetooth®) is active the saved function is available via a short press on FAV.