Maps styled with the Mapbox Studio style editor can be added as basemaps in ArcGIS Online. Any of the preset basemaps or a custom style can be added as the primary tile layer for an ArcGIS Online project.
In ArcGIS Online, start by clicking on Add > Add Layer from Web.
Choose A Tile Layer from the dropdown.
In Mapbox Studio, go to the Style page of the map to add and click on Share & use.
Under Use style in other apps click on ArcGIS and copy the URL by clicking on the clipboard icon.
Paste the URL in to ArcGIS Online. Check the Use as Basemap box, add a Title and add “© OpenStreetMap contributors Design © Mapbox” to Credits and click Add Layer.
The Mapbox style will now be available as the basemap in ArcGIS Online.