I am trying to copy the ID value and register survey 123 though do not understand what they are stating.
Are you attempting to register Survey123 on an older (pre-10.4 for the web, pre 10.5.1)? In that case, please look at How To: Configure Portal for ArcGIS to work with survey123.arcgis.com .
The user item page is a web page reached by logging into your ArcGIS organization (either ArcGIS Online or ArcGIS Enterprise) in your browser. After logging in, click on 'My Content' and you should see a list of all items that have been published. Clicking on an item will take you to the Item Details page.
I registered the app on the page mentioned. How do I use that ID to change the Survey123 app ID? I can open My Surveys though saying I do not have access to this feature yet. Need to figure out how to properly register this in my portal.
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