Q:

TFS - mapping Source Control Folder to VSO directory from on-prem installation?

The Challenge

We have an on-prem application (RM) that cannot communicate with VSO
Our code must stay in VSO
We need to give RM access to Build Definitions that are tied to our VSO source

Attempted Solution

Install TFS locally to host build definitions only
Have those build definitions pull from VSO

Problem

It looks like we can't use a VSO project directory in the Source Control Folder mapping

Is there a workaround?

A:

Your best bet is to start migrating to the new Release Management service in VSTS, since the existing Release Management Server application is rapidly being deprecated. There are tools available to help ease the pain of migration. 
You could also use RM Server in non-integrated mode -- nothing would be tied to build definitions, and you'd have to specify the path to the build drop manually when queuing the release. It can still be automated via the ReleaseManagementBuild.exe utility in the Release Management client folder, it would just take a bit more effort to build it out.
You could also build a custom build process template to pull the code from VSTS and build it, but again, that's investing a lot of effort in RM server.
[Full disclosure: I am a contributor to the migration tool linked above]

