Hello Gavin Wepener,
If you are creating a survey from an existing feature service the repeats should automatically populate with the related tables on the service.
If you are trying to create a survey that matches an existing feature service the name of the repeat needs to match the name of the table from the REST endpoint.
For example in my test survey the name of my related table was TenEightDB.SDE.defects, when I created my survey based on that service that is the name that populated the repeat:
This matches what is present on the REST endpoint:
If you do not see your related table on the REST endpoint it may be likely the table was not present in ArcGIS Pro or ArcMap when publishing the data.
Thank you,
Zach
Thank you,
Zach