Hi, we developed a web app based on the iPhone Safari browser and would like to apply to see if it is possible to use JavaScript capabilities to prevent the iPhone system from taking screenshots of web pages.
There's no supported way for you to prevent screenshots from JavaScripts running in the Safari browser with the APIs currently available. If you'd like us to consider adding the necessary functionality, please file an enhancement request using Feedback Assistant. Once you file the request, please post the FB number here.
If you're not familiar with how to file enhancement requests, take a look at Bug Reporting: How and Why?