As far as I can tell, AGOL doesn't directly support serving up a WMS URL/service:
Could Esri consider adding direct AGOL support, for use cases where we don't want to use Portal/ArcGIS Server, such as this? Serve up WMS/TMS/XYZ layer to iHunter app
I'd be willing to pay for the bandwidth (extremely minimal), if that's a concern. Although, to be honest, web GIS isn't my area of expertise, so I don't really know why AGOL doesn't already support WMS in the first place.
Thanks.
