arcgis-onlinepopupweb-mapping
How can I show an actual web page (rather than just a hyperlink) in the pop-up box of ArcGIS Online?
In the ArcGIS Online "Pop-up Contents" section adding a url in the "Custom Attribute Display" just shows a link, and clicking on it opens the url in different page.
Is there any way to show the web page directly in the map pop-up like happens for images or charts when we click on a layer, as in the mockup below?
I've never used Flex Viewer, but I just went through a similar situation with ArcGIS Online. Apparently, web browsers don't normally allow local links for security reasons. You could try installing a LocalLinks plugin or add-on depending on the browser you're using:
Firefox - https://addons.mozilla.org/en-us/firefox/addon/locallink/
I did not try this method personally, since I didn't want to install anything on my work computer. Instead, I stored all of the files I needed on my company's file share website, which created a web URL I could copy/paste for each file.
You can do the same thing with a Dropbox account, instructions here: http://blogs.esri.com/esri/arcgis/2013/07/17/using-dropbox-files-in-arcgis-online/
This was the best solution I could find without having to program.
The past few days I've had a chance to look further into what potential issues here, here and here may be causing your image to not display as a thumbnail in the configure pop-up in line portion of the dialogue box.
Furthermore, looked at some potential solution here, here and here. The solution range from external links from image storage sites as well as specifics to how the configure pop-up is either to be a hyperlink or a thumbnail.
To get to the solution, I believe the issue lies in the URL that you have been using to display the image in-line within the configure pop-up box. Here is a good example of choosing the image directly through the URL with an extension .png and .jpeg. So you could try to copy the image address directly of the image and that might work within the URL.
An alternative option would be to create a field in your shapefile in ArcGIS Online 
then use an image storage site like imgur, create an account, upload the image 
and use the direct link to insert in the ArcGIS Online 
,
this approach worked for me
.
Best Answer
As noted in the answer above the HTML tag is not allowed in the custom attribute editor. However if you write the tag and the {url} to a field of the feature layer it will be executed by the popup.
I did it by using ArcGIS Online's field calculator and adding the and it's attributes to each row.