How to deploy a node.js app to Azure in Visual Studio Online?
Now the VSO allows the file picker to choose the path to the Web Deploy package.
I am not sure whether Visual Studio Build will create a deployment package for a Node.JS application.
You can try one of the two things.
You can create your own zip package from the code by using the generic powershell task and then put the zip's full path on the agent machine in Web Deploy Package field.
You can always check-in the zipped file which then can be selected using file-picker.