How to access a secured Essentials site Datalink REST endpoint from an external app?

I'm developing a javascript app that utilizes our portal's authentication via Oauth2.  One thing I'd like to be able to do is use an ajax POST to retrieve information from an Essentials data link rest endpoint.  I've got this working using an unsecured Essentials site, but I need to be able to access an Essentials site that's secured using our portal authentication.  Is this possible?  

 

 

For example, I want to access https://maps.orcity.org/Geocortex/Essentials/REST/sites/OCMap/map/mapservices/46/layers/0/datalinks/Permits   However, I can't figure out how to pass credentials to it without manually clicking the Sign In button.

 

 

Thanks,

 

 

Chris