The core scripting for the drop detection occurs in the same button from the previous step. 

In the on (release, releaseOutside) { mouse event choose Set Variable from the actions list.

In the Variable: box on the right type:

../../:debug

To set the value of this variable, click on the Value: field of the set variable. Then under the Functions set of actions, select the getProperty action.
You will now see:

getProperty ( target, property );

We need to replace the text that says target and property with the correct values. The target is the dummy icon clip so we replace the word target with:

../../icon

Next we need to get the _droptarget property of this clip, so we replace the word property with:

_droptarget

The final line should look like this:

GetProperty ( “../../icon”, _droptarget )