I replied to your other post, but for anyone coming across this post, here's how I did this:
1. In the app configuration, go to Interactivity -> Share, and toggle on Sharing.
2. Publish then open the app and select any feature by clicking on it.
3. Click the Sharing button, then Copy Link. This copies a shortened link to the clipboard.
3. Paste the link in the address bar and press Enter to load the app. This changes the shortened link to the full link in the address bar. It looks like this:
https://civiclens-demo.maps.arcgis.com/apps/instant/attachmentviewer/index.html?appid=bc91f23560d9436fa069f28d980db38c&defaultObjectId=14&attachmentIndex=0&layerFeatureIndex=0&selectedLayerId=190ffe37ae2-layer-4¢er=-114.05;46.8919&level=19
4. From my testing, you can just swap out the defaultObjectId value to open the app to the feature of interest. You can also omit the layerFeatureIndex parameter (I'm not sure what it does), but it looks like you need most or all of the others.
Hope this helps.