I have the code from a customised story map that I would like to modify and use on my own story map (I have permission from the author). However, when I go to add the code into my ArcGIS online account I get a popup that says the file type is not supported.
If you have any clues on how I can make this work or other tips on how I might be able to customise my storymaps with code your input would be very welcome!
Hosting app code on ArcGIS Online is not supported. You'll have to host the app on your own server or other web-enabled file hosting service (like Amazon S3).
You can find more documentation on how to deploy a custom story map on the GitHub repo for one of our storytelling apps. For example, here's the README for Story Map Journal:
You also might be interested in checking out our Story Map developers' blog:
Hope this helps -- let us know if you have any more quesitons.