Hi @SaadhasnainKhan
To clarify - this means is that we only allow apps that are build by Esri and hosted on the ArcGIS online platform (i.e. their hostname ends in "arcgis.com") to request authentication information from the host application - in this case a Site.
For security reasons, we do not support this for custom developed applications, and to date we do not have any means or plans (that I'm aware of) for external developers to host applications on *.arcgis.com domains.
Sorry, that's likely not the answer you were hoping for.