Hi there, Alexandra. There a few ways you can do this.
In Compilation panel, click Add content -> Add Layers. Then select "ArcGIS Online URL" from the left column. Then paste the URL of the map. Click the "+" to add the item to the map. By the way, I have found using the URL from the browser better than the "copy URL" button on the page.
The other way you can do this is to save the VT layer to a folder on ArcGIS Online. Then, sign into that org in Maps for Adobe and when adding data (Compilation panel, click Add content -> Add Layers) select "My Content" from the left column, and navigate to the folder containing your basemap.