In the Web Map you usually find the copyrights/credits/attribution for all the layers within the map. This information usually comes from the "Credits (Attribution)" field in the item details.
But there is a special case.
Suppose you are using an external WM(T)S for your basemap, the credits come from the capabilities of the services, which overrules even the information you put in your item details (in case you create an item for the external service).
I needed to to this for the Sentinel-2 global cloudless mosaic – EOX (Check the License chapter on the bottom of the page). The WMTS endpoint combines several other data and has a general information in the <ows:AccessConstraints>-tag of the capabilities: https://tiles.maps.eox.at/wmts/1.0.0/WMTSCapabilities.xml
Esri Technical Support says that there is no way for them to modify the attribution of that WMTS. But I found a workaround:
- Login to ArcGIS Online Assistant
- Choose: I want to... "View an Item's JSON"
- Select your (base)map using the external Service
- Scroll down to the "Data" part of the JSON and click the pen to edit
- Write your attribution in the copyright text between the double quotes in the "baseMap"-Section (see screenshot).
- If you add a <a>-tag, use single quotes for the URL
- Refresh the Web Map and the changes are reflected.
Note: If you have already other web maps using that base map, you need to edit their json manually as well. They are not updated automatically.