Hi Anney
Take a look at this video:
Then select the textInput box that you want to attach the event handler to then right click on it and select properties. This will show the properties associated with the textInput element that you have selected. It should look like the one in the video.
You will see several on properties. Some or all of the ones I mention above will be present. Click the blue add button next to the handler you want to use. Then hit return(enter).
As shown in the video a function will be added for you in the page code section.
This is where you can add the formatPhoneNumber function.
If you are still having problems please share the published url for the page and any forum developer can take a look to try to debug the problem.
Hope that helps
Steve