How can I open a web page in App Inventor

Add ActivityStarter to the screen and configure its properties:

ActivityStarter1
Action:android.intent.action.VIEW
DataUri:http://example.com 

Leave the other properties empty.

On the same screen, configure the event handler of the button to call the activityStarter1.StartActivity:

By pressing the button, the web browser is opened with the page http://example.com