1. Go to the component tag and use content assist to enter the *Function attribute.
Ex: <s:List labelFunction=""
2. Type the function name.
Ex: <s:List labelFunction="myLabelFunc"
3. With the cursor still between the quotes hit Ctrl+1 (or Cmd+1 on Mac).
5. Select it and hit enter. FB should create the function with proper signature and go into linked mode.
For adventurous developers:
If you find the "Create function ..." option missing any particular Function property for any component you can edit the componentDescriptor.xml file (which can be found under the Adobe Flash Builder 4.5\eclipse\plugins\com.adobe.flexbuilder.mxml.editor_4.5.0.xxxxx directory) and add the entry yourself and fix it !
Caution : Save the original xml file in case you end up spoiling it.