Hi Jay. ArcGIS Online Web Maps do not support joining Feature Services from Online and ArcGIS Server. However, you could download your data from ArcGIS Online and then perform the join on the extracted data using ArcGIS Desktop. If you follow this approach, new data added from Survey123 into your ArcGIS Online feature service will not show in your join until you download and join again... but it is the first workaround I can think of for your use case.
With the December 2016 update to ArcGIS Online, a new Join Features tool is now available. If your ArcGIS Server Map service is exposed to the internet, you can use the service as valid input in the tool.
Here is the documentation about the new tool: Join Features—ArcGIS Online Help | ArcGIS