add a transparent button/movieclip in a keyframe above your tween where you want to apply your code and code for that transparent button.
You should be able to apply Code snippets on a symbol irrespective of the Motion tween.
Just goto the first frame where the Symbol is present(beginning of the Motion tween), Select the Symbol on Stage and apply the required Code snippet.
If you still face issues, please provide a snapshot of the timeline and stage after selecting the desired symbol.
This looks fine. You should be able to add code snippets at this stage.
Try following the steps below:
1 Select none (Click somewhere on the empty region on the stage)
2 Click on Frame 1 of your third layer. (make sure only one frame is selected)
3 Open the Code snippets panel, choose the desired code snippet and apply it. (the code should get added Actions layer.)
If you are writing the code manually, select Frame 1 on Layer 1(Actions) and then add the code there.
Hope this helps!