I recently switched our aerial imagery provider to Metromap. I have an enterprise subscription which uses Microsoft's SSO to login users to the web app which has worked great. To use within Portal for ArcGIS I created an API to the WMTS which works great in portal. Note "Austalia_latest" is the API layer.
When the same map is accessed via one of ESRIs apps such as Collector I get the prompt "Secured Basemap". Thing is as I use an API so there is no username password.
I reached out to Metromaps support but haven't had a resolution. My field workers are currently having to cancel the prompt and switch to a different basemap. Has anyone else experienced this?