If I understand you right, now you just need to capture a partial screen.
Remember that the screen size might not be the one required by Apple so you need to fix it with one of the online screen converters.
Alternatively, you can use one of the apps I shared in an earlier post.
[Update]
Here I have chosen iPad Mini and viewing a site registration
This is a screenshot of a sample app
When you start your app from the Live Test menu button
You need to click Preview as a responsive web app link
Then you can right click and choose inspect.
This is an iPad Mini version of the sample app
