Support > Advanced Web Page Options > JavaScript Snippet
This field allows you to specify some JavaScript that you would like the browser to execute before capturing a screenshot. This will allow you to program the browser to remove a dialog, click a button or submit a form before capturing the screenshot.
The best way to test this is to first run your JavaScript on the web page from your web browser's development console. Once you have validated that your JavaScript is sufficient, copy and paste it into this field.
Javascript Snippet works on a single page. If you have more complex scenarios, like logging in to a website before taking screenshots, use the automation steps instead.