Hello there!
I'd suggest starting this workflow by reviewing our online documentation.
To make custom boundaries available as reference layers in ArcGIS for Excel, upload the shapefiles (packaged in a .zip file) or GeoJSON files (.geojson or .json) to ArcGIS and publish them as hosted feature layers. Then share the layers.
To learn how to publish hosted feature layers in ArcGIS, see Publish a shapefile and Publish a GeoJSON file in the ArcGIS Online help.
Once you've attempted the recommended workflow, please reply here and let me know which step you may be getting stuck on.
Here is an additional resource from our ArcGIS Online documentation that really provides a nice overview.
All the best,
Shannon Cardoza
Product Engineer