For clarity, this is what I’m after...below an example of the FME Flow ‘Draw Area of Interest’ tool with a an AGOL public web map embedded. The blue polygon (e.g. feature hosted layer) shows the user the extend of the data available to download,

I have done research but can’t find an answer on how to ember AGOL webmaps onto the FLow Map configuration...

I am not really familiar with how AGOL generate web services, but geometry type parameters only accept public WMTS URL for the basemap. The URL must look like this : https://<service_url>/{z}/{x}/{y}

So you’ll have to create an WMTS URL from AGOL. The following link might be helpful : 

https://enterprise.arcgis.com/en/server/latest/publish-services/windows/wmts-services.htm

Thanks ​@alexbiz for the info,

Unfortunately I think isn’t possible to embed AGOL or Portal WMTS into the Map Configuration. I have checked the link you suggested and also looked into the following: 

https://support.safe.com/hc/en-us/articles/25407744364941-Using-the-Geometry-Parameter#SetCustomBasemap

‘How to Set Up a Custom Basemap’

...but couldn’t get to work the {z}/{x}/{y} in my WMTS URL (which is public).

I think for the time being I will use the default basemap or potentially use one of the public tile layer services suggested by Safe.