Kris,
It's been a while since I last used it and my own example have been deleted, so I'm just copying from my old notes. Assuming you publish the FC/table with relationship and add the service to a web map (which will be used in Collector). The URL part is done on the web map via "pop-up" configuration of the parent ("asset", using my "asset" and "Inspection" example).
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This was the link in my test:
arcgis-survey123://?itemID=ba6b648f7ee64147899e5f3f67bd8d1e&field:parentID={GlobalID}
In this case “parentID” is the GUID field of the “Inspection” table, and “GlobalID” is the GUID field of the “asset” feature class, and they are what’s used as foreign key and primary key respectively in the relationship class.
The itemID in the “Link” can be obtained from ArcGIS Online when you clicked on the form item, e.g.:
Hope this helps.