I want to know how to trigger an event

How do i trigger an event?

what kind of event you want to trigger. there are multiple ways to do it.

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An event block

What about elaborating a little bit?
For example the Web.Get method will trigger the Web.GotText event

EDIT: after having seen your other thread here How do you add properties? I now understand you are talking about developing an extension…

Just check the MIT App Inventor sources… there are loads of examples about asynchronous processing and how to trigger an event… see for example the web component…