you're probably wondering why your post doesn't get any responses.
I think the reason for that is that your question is very specific, while at the same time, not explaining what you are talking about.
- What App ID?
- Where is that App ID needed?
I'm guessing you are talking about the ID property in the About tab in the Project properties.
Have a look at the Construct 2 Manual, it usually helps you with C2-native properties.
Extracted from there:
An ID uniquely identifying your application. This should be in reverse domain format, such as com.mycompany.mygame. Some exporters use this as the ID for your exported app, so try to ensure it will be unique.
This should answer your question.