The workflow (Open in ArcGIS Desktop> Open in ArcGIS Pro) within ArcGIS Online does not automatically add to the Contents pane in ArcGIS Pro 2.3.0, but it automatically opens in the Table of Contents in ArcMap 10.6.1.

Workaround

1. Navigate to ArcGIS Online.
2. Enter the username and password for an organizational account.
3. Select Content > Overview.
4. Find a web map, and select the corresponding three ellipses icon to view the item details.
5. Select Open in ArcGIS Desktop > Select Open in ArcGIS Pro.
6. An item.pitemx file pops up. Select the file once it is downloaded. The web map does not open.
7. In ArcGIS Pro 2.3.0, open the Catalog pane, and click Project. In the Maps folder, select the web map from ArcGIS Online.

Steps to Reproduce