You can publish services directly from ArcGIS Pro to ArcGIS Online by following these instructions to create a hosted feature layer. With this method you don't need ArcGIS Server.
If you mean register rest services with ArcGIS Online that you have published to ArcGIS Server, follow the documentation here for registering services. Using the REST endpoint, add it in your content by choosing add item>from a URL.
If that doesn't answer your question, I would recommend posting your question in either the ArcGIS Online or ArcGIS Enterprisecommunities. You may be able to get more eyes on your question and possibly an answer there. This group is for the productAdmin Toolsby GEO Jobe.
.png "MichelleBrake"){kind=avatar}
