I’m definitely not an expert at Javascript so I might not be the best person to help you but the general idea is to create a script and upload it to your project. Then, you assign the script .html file to the Web Viewer as its url. And finally, you use the Web Viewer message event blocks to send and receive data from the script. I know that’s just an overview… I wish there was a full tutorial on this but I’m not aware of one.
This is a pretty good explanation: GitHub - thunkable/webviewer-extension: Communicate between the Thunkable Web Viewer and a website
And a couple other useful links: How to send/receive a message using js - #2 by jaythemanchs and Date operations (No API) - #3 by muneer
@domhnallohanlon I saw an old tutorial video you posted about this process. It would be great to have an updated version!