Hey Andrew,
We do not have a direct way to write to a portal, yet. The workaround would be:
1. Write feature as a feature class in a gdb using arc.write
2. Use the Share ribbon in ArcGIS Pro to share that feature class as a feature service.
I know this is not an automated approach, however. If you host data frequently you can perform the above step once and put the Share step in a scheduler in Pro.