Map Viewer does not support adding Web Map Service (WMS) layers with the CRS 3301 projection as a basemap when Esri default basemaps are utilized.

Workaround

Workaround 1

1. Create the web map with the OGC WMS layer as a basemap in Map Viewer Classic in ArcGIS Online.
2. Save the web map.
3. Open the web map in Map Viewer.

Workaround 2 

1. Open Map Viewer Classic in ArcGIS Online.
2. Click Add > Add layer from Web.
3. In the dropdown, click A WMS OGC web service.
4. Paste the URL of a WMS OGC web service.
5. Check Use as Basemap.
6. Click Add Layer.
7. Save the web map.
8. Add this map to a custom basemap gallery group.
9. Open Map Viewer. Adding a different OGC WMS layer in CRS 3301 displays the layer properly.

Steps to Reproduce